My post is very long (too long), but I really hope you find it helpful:
Your son sounds a lot like my 8 year old, though it sounds like you are having a rougher time and maybe not getting as much support.
Our son went to the dentist for a few years before the dentist recommended we take him to another dentist. Fortunately, he had a specific dentist in mind. This one specializes in working with kids with special needs. The staff is amazing! They have lots of experiences and lots of tools in dealing with different diagnoses and different behavior problems. I was shocked and impressed at what they were able to do with our son!
Talk to your dentist and your pediatrician and yoru psychologists. There really are experts int eh field who know how to work with your son. It is so easy to feel hopeless and overwhelmed and just completely discouraged. The right people for your son do exist! Keep looking for them!
Someone else mentioned finding another school. We are so thrilled that this was an option for our son. Our son had huge behavioral struggles at school. He was getting in trouble everyday, and the teachers didn't know what to do. The special ed teachers tried to help, but it just wasn't working. The special ed coordinator let us know about another school that specialized in behavior problem and special needs. They have a "Structured Teaching Program." My son is in a class of 8 to 10 kids, with 5 adults (1 teacher and 4 aides). He works with an adult one-on-one and then does independent work in his cubicle. They use a work, break, work, break schedule. Their PE class is very small, and they eat lunch in a lunch room with just one other class. Many of the kids have sensory issues, so these small numbers are a huge help. The school uses a "wait it out" approach to must behavior problems. If a child refuses to complete a task, an adult will simply wait for them to do it. My son refused to do something one day, and his teacher patiently waiting. He began yelling at her, and she just stood their waiting. He got frustrated and said, "Are you ear blind?" She told me she had to turn around because she couldn't stop laughing. But it was only a couple of minutes before he decided to go ahead and complete the task.
The principal of our son's new school told me, "I only have to wait one more minute than he does," and that gave me a whole new perspective on my son and his behavior. We were often told to just wait him out, but when you're trying to get out the door or when you're standing in someone's way or when you're at the grocery store, the idea of waiting them out can be terrifying. But hearing her say that helped me to see that I really could do this. I began trying to wait him out. I did my best to stay calm and prepared myself to have to wait for quite awhile. The first couple of times I dug my heals in and chose to wait were difficult and seemed to take forever. But the taught my son that I wasn't going to give in. It wasn't too long before his refusals started to decrease. He really started to understand that I wasn't going to give in and that maybe what he was being asked to do wasn't so bad.
I would definitely contact your principal and the special ed coordinator. Ask them about alternatives. There might be another school that would work better for your son. I know lots of kids have special needs and are able to receive accommodations and be in their regular classroom, but that was just not working for our son. Having him go to another school has been the answer to our prayers. They really know what they are doing, and it's a much, much better environment for him. I think it's something you need to explore.
I really encourage you to talk to all your doctors again. They need to realize that you are feeling quite desperate and possibly hopeless. They need to help you find better resources. There are behavioral specialists out there who can make a difference. Family counseling might help. We've gotten some great tips from family counselors over the years. If your son is really refusing, you might look into ABA therapy. Oftentimes they will come out to your house to work with your son in his environment. Our son was initially turned down for ABA by our insurance company because he didn't have an Autism diagnosis. He does now, but at the moment we are happy with his current therapies. We might look into it again at another time.
I don't know anything about residential programs, but that might be something you do need to consider. Please do talk to your doctors. They need to help you find resources to help your son.