Prosthetic leg...........Hmmmm. My grandmother told me, many years ago, about a 'banty' chicken that had it's leg frozen off one winter. They took a clothspin and clipped that to his leg and it worked well. Now, I'm not necessarily suggesting you use a clothspin, but I wonder if we couldn't put our heads together and come up with something? Some kind of a small pill cap or small cup, padded, for the stump to rest in, with maybe a velcro type strap to attach to the leg, and attached to a metal pin with something smoothe at the end that touches the ground. I'll have to think a bit.
Although, he may do just fine with his stump!