I would also suggest stretching. Lots of it. ...
Also, see a podietrist. Make sure the mechanics of his feet are okay and he doesn't need orthodics. My knee issues screwed up my feet. I have psoriatic arthritis in my feet as well and between the 2 issues, my feet became all messed up.
Stretching, orthodics, meds that actually did some good (currently that med is Plaquenil) and time - it was a looong road, but I made it.