Years of mistakes... Is there still any hope?

So, I bought my Quaker Parrot ~4 years ago when he (could be she) was 8 weeks old. I, unfortunately, did not take the time to train him as much as I should have. He loves me and rarely bites but he HATES everybody else... unless they are giving him treats, otherwise it's flying attacks. Nonetheless, all I really did with him was teach him to step up, teach him let me open his wings, and pet him on the head. Most of the time he just chills out, sings, talks, and sits on my shoulder while I do whatever I need to do around the house. Harness training was failure and he hates being outside. He gets scared.
I don't have hours and hours of the day to spend with him, especially because he's not allowed out of his cage while most other people are home--because of the flying attacks. One of the unsavory individuals in my household even threatened to smack him across the room and even kill him if he ever flies on him again.

Nonetheless, I've changed my ways and I've been trying to spend more time with him and to train him to do new things, like have him sit happily on his perch or on the couch or on top of his cage without him always needing to be on my shoulder. I think this will help me potty train him because I can catch him in the act better. I think he's mad at me because I don't let him sit up there as much anymore. Anytime I try to teach him something new, he flies back to his cage. He want's nothing to do with learning new things. He feels that if he's not on my shoulder he would rather be in his cage. He's afraid of most things I introduce to him too. I don't want to clip his wings because I'd feel bad taking away what he's been able to do for the past three years, plus I own a dog. I'd like for him to be able to get away if something were ever to happen. Plus, I think it's good exercise for him. They were clipped the first year I had him, but I felt sorry for him and let him learn how to fly.

Oh, I have target trained him. He knows how to bite a chopstick. He doesn't like to be touched/handled all over though. He shakes his head and prepares to bite if I do anything besides pet him on the head or open his wings; so, I leave him be. I don't want to force him to do anything. He usually always get's his way because I don't want him to get mad at me.

My question is: is there any hope for him to learn new things and accept new experiences, like to be off my shoulder? Is he set in his ways forever because he's done the same thing for 4 years now? I'm starting to get discouraged by him getting grouchy and his fling to his cage whenever he doesn't get his way. I don't want to do something wrong and stress him out and have him hate me for the rest of forever.

I want to teach him to like being outside with me too. But since he flies and doesn't like his harness I keep him in a cage when I go out. He hates this because he doesn't get to be on my shoulder. How can I train him to be by himself?

Is there any hope or should I just stop trying before our relationship is severed forever?

No, he doesn't have to be set in his ways. Birds can learn new behaviors, no matter their ages. Of course, the longer they've been allowed to indulge in bad behaviors is the more deeply ingrained those bad behaviors will be. But all that means is that you'll have to be that much more patient and consistent with the training process.

He's going to be annoyed at your attempts to change the dynamics of your relationship with him. He's always been able to hang out on your shoulder with complete impunity, for instance, so of course he'll resent not being able to do so now. But annoyance alone will not sever your relationship with him. Loss of trust will. Which is why consistency and patience will be key to the process.

Seems to me the first hurdle you need to overcome is Pingo's tendency to engage in flight attacks. This particular behavior is inhibiting your ability to address any of his other issues, as it limits the amount of time you can spend with him outside of the cage and makes socialization impossible. In fact, it's a self-perpetuating behavior. Parrots are highly intelligent. He almost certainly has picked up on the fact that he gets locked away from his favorite person whenever other people are around, which reinforces his perception of anyone other than you being the enemy.

Like you, I LOVE the idea of keeping my birds flighted. But I've always said that if there was one thing that would get me to temporarily clip my birds' wings, it would be for a safety issue. For either the bird himself or for everyone around him. And it sounds like, given the regrettable attitude of at least one of the members of your household, Pingo qualifies for both. So much as I hate the thought, (not a criticism of clipping. Just my personal preferences, here.) I would probably consider clipping in your situation.

But you mentioned the safety concerns of clipping since you have a dog and you'd like for your bird to be able to escape if need be. (It always makes me nervous when someone lists that as a reason to keep their birds flighted. Indoors, your birds are at a severe disadvantage. Your own imposed safety measures should be enough to ensure your bird's safety from any other pets, regardless of whether or not they are flighted or clipped. A dog is just too fast for a bird's ability to fly to be a significant factor in maintaining his/her safety. In the blink of an eye your bird could either be gone or permanently maimed. But I digress...)

So if clipping just isn't feasible, or you are strongly enough against it as to render it unacceptable as a choice, you're probably going to need some help. Someone (preferably someone agile!) who is willing to work with you and Pingo in a closed room. (A different room from wherever his cage is kept! Just in case there are territorial issues at work, here.) A room that is relatively unfamiliar to Pingo would be preferable, btw.

Bring a training perch into the room with you. Two would be even better. Pingo is target trained, which is good. Have him do some targeting exercises with you, (which will hopefully distract him from flying over and eating anyone) but let your "assistant(s)" be the one(s) to reward him with his favorite treats. The idea here is for Pingo to begin associating other people with the potential for treats and all thing tasty. But if he decides to attack, you must be the one to say "no" in a firm but even voice and then take him and put him on timeout. Timeout can be on the training stand and you can either turn your back on him for 5-10 minutes, if he's the type that would stay there or only have to be returned once or twice, or, if he's a skilled enough flyer that he would just keep flying to you repeatedly, you might have to leave the room. (While watching him through the cracked door, of course!) The point here is to deny him the pleasure of your attention for the duration of the timeout. (If he's too curious for the out of the room idea to work either, you can just use his cage for timeout.)

It will take a lot of repetition, but eventually he should start to get the idea. As he becomes more reliable around your assistant (which might be over the course of days, weeks or even months) you can progress toward having him/her interact with Pingo. Also, if Pingo's attacks only happen when you are around, you might want to try having your assistant work with him sometimes without you in sight.

Once he becomes more socialized, you will be in a much better position to work with him on other stuff. Hope this all helps.

Start over from square one.

Treat him as an untrained bird, and Re-train him the right way...

Reinforce the basics. Set the boundaries. Get on with your life little bird...

Chances are, as his situation improves, and he gets interacted with daily, he will.