Maybe you could try going to another solicitor? They are not all the same. Shop around and see what you can come up with. I think it may be worth having a try. They cost a lot of money so don't be afraid to ditch one if you are not getting value for money.
What contact do you have with your son now? You'd probably need to demonstrate radical change in the way you had come across in court the first time round in order to change the order. Whether you are right or wrong the judges expect you to do things their way.
I know you'll probably be unhappy with me saying this - but do you really think he would cope in a mainstream school? At his age my girl was debating views on religions etc and starting maths equations. Could he realistically take full part in lessons? I have no clue what he is mentally capable of but my observations of a child with severe autism who was in my girl's class in junior school were that he was taken out of most lessons in years 5 & 6 as he could not manage the work and was becoming increasingly stressed. He then worked on his own with a classroom assistant, who was only part time. His parents (mom is my friend and shared her worries about him with me) took him out of school and got him into a specialist unit where there were adequate staff who were specially trained to cater for his needs.
I'm no expert but I reckon your boy needs appropriate time with appropriate people. I totally understand that you want him to lead a normal life but mainstream schooling may not be right. My daughter is 13 and I assure you that the work has got more and more demanding as she gets older! We're into chemistry experiments, esssays and three languages now. She struggles at times because of the pressure. She's not particularly clever and has to work hard to achieve. Hormones kick in at around 10 or so as well and add a whole new dimension.
However it doesn't sound as though he has the right provision now. Endless play is simply not appropriate. My experience of people with Down's is largely of those in a special needs school. They were taught life skills and went on to have jobs and were largely very happy.
It's understandable that you want to protect yourself from your ex but maybe you need to bite the bullet and try and establish better terms with him for your son's sake. If he is struggling then it's better that he can ask for your help so your son doesn't suffer. The roles are completely reversed now and your ex will be understanding more about the role you play in your son's life.
Your contact HAS to be balanced, stable and appropriate though. Courts take a very dim view of people who don't comply with their orders! Do absolutely everything you can to comply with the order and you will find it easier to change in the future. Fight it and you will slim your chances.
Hope this helps a bit...