Hello, sorry to hear about your son. I to have the condition and I was diagnosed at the age of 2, because they caught it early they were able to put me in a half body cast which helped me while I was younger. The cast helped with the density of my bones around my joint without putting any pressure on my joints. Doctors have told me that the cast has held off a lot of the pain and symptoms during my youth and I was able to be a "normal" boy. I would search your area for a good doctor who knows about M.E.D or that are willing to learn about it, don't be afraid to speak to a few different doctors until you find the right one.
I hope that helps you even a little.