I echo janey's sympathy, my son is a similar age to yours(he is 11) and has problems with many joints slipping, giving way and completely dislocating. When he has actually injured a knee in the past we bought a sports knee support from Boots, which had a hole in the middle for the kneecap, and hard splint type bits down the sides, he wore this in the day to try to manage to walk about, but we took it off at night. This was just for a few days while his knee recovered. I wonder if the physio is right about a permanent spport weakening the muscles though, this can happen and it might ruin all the hard work done by the physio. Ewan has daily physio, it is done in school by the teaching assistants. We have had good support from the school and education authority, especially since getting a statement of Special Educational Needs(SEN) this is for his physical needs, as he has no learning difficulties.(sometimes people think that kids have to have learning difficulties to get a statement, but special needs can be anything that makes it difficult for a child to access the national curriculum).I have written a lot about school and help under various threads to do with statements, school etc, have a browse, or pm me if you have any questions I may be able to help with. The school are understanding about attendance issues and he has various aids to help him be more comfortable in class such as a special chair etc.
Painkiller wise have you tried codeine? If he is unable to take ibuprofen it may help. Ewan takes diclofenac (voltarol), paracetamol and top ups of codeine if he has particularly bad pain or injuries. Wheatbags heated in the microwave are useful and hot baths to relax the joints. In the night he sometimes listens to a relaxation tape which he finds soothing, and I also do reiki on him.
Hope some of this helps
Aged 47, dx EDS 3,POTS, Coeliac, breast cancer 2002. I have 4 children: son 14 - HEDS & Coeliac, daughter 25 - HEDS, POTS, pernicious anaemia, daughter 27 - HEDS, Behcets syndrome, also daughter 22 & 3 grandchildren unaffected as yet.