I myself have had surgery to try and improve my OSD pain. I'm 20 but got diagnosed when I was 12. Although my pain was quite bad, espiecially when doin/after sports it wasn't as bad as what you son seems to have.
This is the first time I've heard of someone having to use crutches because of it.
I had my operation last year (I only have OSD in one knee) to remove the "lump" below my knee. I was never placed in a cast but was bandaged up for a couple of weeks. within 4 weeks I was walking around with no aids.
My doctor was very wary of giving me surgery, espiecially at a young age, I was "lucky" to have them agree to it at 19, never mind at 15 like your son.
Unfortunatly the surgery wasn't succesfull for me and I get the symptoms are the same, if not worse than what they were before. I am now awaiting another consultation for a second oppinion.
I really feal for your son, at such an active age aswell it must be really difficult to be immobalised and to have surgery on both knees at once.
I notice this was posted about a year ago so I really hope that it's all been cleared up now. It'll be great to hear how it's turned out.