Awe, that's a terrible thing to have happen, but Greys are so intelligent, they have the intelligence of a 4-5 year-old human toddler. They are also creatures of habit. So not only was he just taken from his home of the first 5 years of his life only a few weeks ago, and taken away from his "people" from the first 5 years of his life, and then put into a completely new home with a completely new person, then only a couple of weeks into this new home with the new person this very traumatic thing happened to him (his perception of what the Vet did, not that it was really a horrible thing that was done to him, just so you understand what I'm saying)...That's the way that he's thinking about this situation right now.
The other thing is that you were just getting into the "honeymoon period", the first few weeks to months of owning a new parrot, where everything is new and great and wonderful, and also the time when yes, you are first earning your bird's trust. And even though I always recommend that people take their parrots for a Wellness Exam when they first bring them home, I'm starting to wonder if it's not a good idea, especially when it's an adult bird that has just been moved from one home and person to another, to wait a month or two before taking them to the Avian Vet...It might not make any difference at all, I don't know, just an observation...Especially with something like a beak-trim...
Just an FYI, I've know people who had a Grey, a Macaw, a Cockatoo, etc. for years, since they were just weaned, they were their bird's only owner, and who had taken them to the same Avian Vet many, many times over the years, for injections/blood draws, exams, cultures, x-rays where they were gas-sedated, etc., and they had no issues at all at any time...BUT, when they took their birds to the same Avian Vet for a beak-trim where the Vet used a Dremel-like tool to trim the beak, this caused HUGE ISSUES with their bird instantly behaving just like your bird is behaving. I know that it's a necessary thing, beak-trims, as when they get too long they have trouble eating, along with many other problems...but I just absolutely refuse to have my CAV do it. I don't have a bird as large as a Grey, but I have a Senegal who's beak does have a tendency to grow a bit on the long side, and I just absolutely refuse to let my CAV do it with the Dremel tool...My Senegal has all kinds of mineral blocks, cement perches, sandpaper perches, and other such toys that are meant to keep both his nails and his beak trimmed...I do all of my bird's toenails myself, and I also will file their beaks with an emery-board when they get pointy...But after hearing all of the horror stories about the beak trimming with the Dremel tool it's just not going to happen...I actually read an awful post, I believe it was on this forum, where a perfectly healthy Eclectus was taken by his owner to their regular CAV for a beak trim with the Dremel tool, and the Eclectus started breathing in a very labored way during the trim and for a time afterwards, and they put the bird in an O2 chamber for 20 minutes, and he seemed fine, so the owner put him in the car and they drove home...But as they approached home the bird started vomiting in the car and breathing rapidly and in a labored way again, so he pulled the car over and called the vet back and was told to bring him right back, so he drove as quickly as he could with his only 3 or 4 year old bird in his lap, and the bird died as they neared the vet office...Apparently the bird inhaled a bunch of the dust/particles from the beak trimming...this sounded to me like a CAV that didn't know how to properly do a beak trim with one of those electric trimmers, but I don't know much about them to be honest...
Anyway, to get back-on-track with your current issue, as already stated the only thing that is going to help you earn your Grey's trust again is time. You basically need to approach this like you just brought him home today, like he has just left his home of 5 years and his person, and has just been brought into his new home with his new person. The only difference this time is going to be that you are going to have to actually earn his trust first this time before he is friendly with you or allows you to touch him or hold him....So you need to start completely over with the basics.
So as of now he won't even step-up for you I assume? If not, that's where you need to start, getting him to simply step-up for you. That's always step #1, because once a bird is willing to step-up for you, then you are on your way to earning their complete trust. Once he is willing to again step-up for you, ONLY THEN can you start to work on being able to pet him, scratch him, or hold him/hug him again. It will happen, don't lose faith or get too frustrated with him, as it's going to take time...This is always a marathon and never a sprint.
He's had what he perceived as a very traumatic, scary, possibly painful ordeal happen to him, and he is blaming you for it because you took him there (if the vet wasn't the best at what he was doing, I hope it was a Certified Avian Vet that did the trimming and not an "Exotics" Vet, as their beaks have lots and lots of nerve-endings and a blood supply, so if they aren't very experienced at using an electric Dremel tool to do beak-trimmings, they can really cause them a lot of pain)...So you need to just spend as much time as you can spend with him every single day, talk to him as much as you possibly can in the softest, most loving voice that you can, and go very, very slowly. Don't just try to pick him up or start petting him, but rather go very slowly, talk to him, and use his favorite treats to work on getting him to simply step-up for you. And once that starts happening then you go from there...
I forgot to ask you, out of curiosity, were you in the room with him when the beak and nail trimming happened? Could he see you/hear you while it was going-on? If so, that might be the reason that he blamed you for the entire ordeal, that's a tough call to make, while it's so difficult to let them in with the vet alone while they are going through things like that, sometimes I think it's better to let the Vet be the bad guy...