Littlebit,
I am a right above knee amputee - so I can't speak from personal experience. But here are some thoughts:
1. If this man is very strong in his left leg/knee, he may be able to mount normally. I would suggest looking into some of the mounting aids: they have some stirrup extenders, and/or stirrups that drop down 3 inches. That would help. Some of the links to these products are in the Tack & Equipment Forums and in the Resources Forum.
2. Another helpful aid may be the attachments that turn the western stirrups. Also listed in the same areas.
3. The shorter/lower the horse & stirrups, the easier it is to mount. If the equipment additions don't solve the problem, another idea is to train the horse to 'park out'. That would bring the stirrups down another few inches.
4. If he is unable to mount using the left/prosthetic side - he could learn to mount from the off/right side. It would take retraining on both his part and that of the horse, but it is certainly do-able.
Hope those ideas help. I'm sure some others will have suggestions. Please let us know how it turns out and if you have any other questions or helpful advice.
Let your client know that we're rooting for him!
Ann
RAK amputee, Michigan
Western/Trail riding
Spotted Draft Horses