Hello and welcome from Iowa!
Congratulations on your new cockatiel! You are right, it is usually better (and actually cheaper) to get birds from breeders. Usually they are also hand-fed and are already tame and like people. But that doesn't mean yours will never be tame, it will just, like you say, take training and time. It is not only your job to train him, but also to build a trust bond.
As he is still adjusting, you will want to work slow on this. Open the cage door and allow him to come out on his own. if he chooses to hang out on top of his cage, that's cool, let him hang out at his own pace. If he decides to fly around, be careful that curtains are covering windows and that there are no open doors he could fly out of. He will probably be fearful of you still, so just pick a chair near the cage and wait for him to come out while doing another activity - reading, working on a laptop, etc. To get him back in the cage, you will likely have to use a towel to cover him and gently place him back into the cage so he can't bite you.
For diet, the best option is to cut back on what he is normally used to (seeds) while incorporating new foods mixed in - fresh fruits and veggies, cooked beans and pasta, etc (check out my
Chop Chronicles thread here for a list of what I feed my two cockatiels, the pictures don't show up but the recipe is easy to follow), there is so much variety they can enjoy, and sometimes it is just presentation of foods, smaller birds like cockatiels should be fed smaller pieces. My birds also love spray millet as treats. As you decrease the amount of seeds given, then you will likely find better success in giving him seeds as treats (from his old diet) and he will want them. You can offer them through the cage bars, eventually working your way up to offering him treats with your hand inside his cage, and then gradually working to train him to step up. It is a constant battle with the diet, though. Some birds are just seed-junkies!
It is likely he may not know how to fly, as you said he is clipped. They will grow back in but until then keep an eye on him whenever he is out of the cage and make sure the ground isn't too hard in case he tries to fly. He should have a good amount of perches in his cage of different types. I have one concrete one (its trims their nails), one soft twisted rope one, and two wooden ones along with three-four toys.
Right now just try to keep every single interaction with him positive. Work at his pace to get him comfortable with you and your family's presence. He's just been through a lot of change and will need some time still to adjust, so just talk softly to him, sit in the same room and work on getting him more comfortable with you in the room, eventually you should be able to approach his cage without aggression and should be able to handle him. It will just take time and patience! GOOD LUCK!