AFAICT, immobilization is the first step. We didn't go into actual treatment options yet - he's still not 100% sure what the problem is. We just figured that immobilizing the joint probably wouldn't hurt, as it has been getting worse over the last few weeks.

After that, I think it will depend on whether it's a structural fault in the foot (and the extent of it) or just years of mis-use. Since I could still walk on it, he's leaning toward an orthotic.