the summer shows at my college did Irma Vep with a Nicodemous that had a "peg leg". The actor basically had a humoungous platform shoe that he walked in that had about 8" of wood from the bottom of his foot to the floor and the rest was strapped on with some creative costuming to make it look like a leg was missing. (Vep is notorious for the number of quick changes, etc. and involving a wooden leg with a bent knee would be near impossible.)
Anyway, I'm working with that actor on my current project and he is still recovering from some pretty major back issues gained from hopping around at such an odd angle. If you have the financial capacity to get a rehearsal set in place with a rake, I would suggest hooking the actor up with some massage therapy or chiropractic help.
Leaving the peg as an option (not requirement) in rehearsal is best, but if he's overzealous he might do some damage. Maybe it's worth it to establish a schedule with the leg? Sounds stupid right? This gets to that experience that I saw happen with the other actor. He didn't mention the pain/discomfort for fear of being the spoiled apple or diva. I think making sure the actor's health it taken into effect by asking him what he needs before the actor requests it would be a good step. (Not that I'm saying anything new or earthshattering there.)
I think the best bet is preventative therapy before pain becomes a problem. Knee pads are a great suggestion, but it sounds like the problems might be larger depending on the angle of the rake.
Insisting on railings for the steps and maybe an extra costume piece like a cane to give the actor a third point of contact might help make him feel safer.