Wow, it's hard enough to handle your personal emotions whenn you first receive a diagnosis. I can't imagine having to process your own feelings, meet your families needs, and deal with your parents all at the same time.
It does sound like they are acting out of love, dealing with his diagnosis in their own way. Maybe a family meeting with all of the adults would be a good idea, make a game plan, discuss your priorities. Give yourself time to think through how you would like your parents to be involved and outline that for them. If you do it in a way that makes them feel useful, not criticized, they could become invaluable to you. You could even give your mom responsibility for one physical therapy session/activity daily, at a set time, if you are comfortable with that.
We lived with my husbands parents for about 8 months, and my daughter became extremely ill. We ended up working together as a team to care for her and my older son, who is my special needs child, and although rocky at first it became a wonderful bonding experience for us once we figured things out.