I had a very similar case last year. Boy was 4 but the size of a 6 year old and solid as a rock. Very heavy indeed. He did have developmental delays, but in addition to being babied and having everything done to him at home. At the end of the day, it doesn't really matter what the cause is, only that he needs to be toilet trained. This is just another case of having one way of doing things and trying to make a child conform doesn't always work and it is up to the caregivers in the child's life to make accommodations for that.

My first suggestion is to have a sit down with parents. They need to be 100% on board and promise to do things your way all the time. I'd even be inclined to have them sign an agreement and if they dont follow it then termination will be the consequence. I understand this is more work for you, but it is in the best interest of the child and the only way this will go in the right direction.

Secondly, you have to make it work. He is a big kid for sure, which I know first hand the limitations in training a late starter. Buy a potty chair, I can recommend one, that WILL accommodate a child of this size. Will he be 100% comfortable, probably not, but it will do the job. I think if the other kids are on a potty, he should to as it is what is modelled to him, not the toilet. Put him in underwear only so there are no pants to pull up and down or zippers etc. I know lots of people thing he should have this set of skills mastered first, but that is not the kind of situation you are working with. Once he has mastered peeing on the potty and gotten over his anxiety, then you can had some simply shorts with elastic waistband and work up to doing more. You have made the move to start potty training and you have to continue. Is he ready, physically for sure, mentally absolutely not from what you have said, but it does not mean he can not do it. Just gonna take a lot more work from you and parents.

Have zero expectation beyond sitting on the potty and reward him with lots of praise for that small step. This child obviously needs a ton of positive reinforcement and consistency all round. When he does pee, make a big deal and give him a sticker. I used to let the older boy I trained look through the stickers and choose one while he was on the potty and if he had a success eh could stick it on his sheet on the wall, if he did not he had to put it back in the bag. It's not enough to make them a promise, at this age they have to be able to get so close to it and realize that they will lose it should they not produce.

Stop asking him to go, and tell him to go. Not negotiable at this stage, there is gonna be a little bit of conditioning before independence comes. You need to be the one in charge of potty action. If he cries, ignore him but makes sure he stays on the potty, ideally until he calms down. I bring the potty into the play room so I can keep an eye on all the children and often the distraction of the other kids playing helps too. Like I said, zero expectations. First goal would be to simply have him sit on the potty without being upset. Once he stops being upset and anxious the pee will come.