There is no shame in admitting a horse is too much for you. Training horses is trial and error, and when you have someone more experienced do the work for you its more trial than error. Send him to a trainer that will let you come watch regularly, you can pick up some good techniques this way. I also agree, if you have a physical disability that prevents you from effectively doing something you should not do it. I have a physical disability with my hands that prevents me from long lining(cant hold the lines and the whip at the same time) so i found a different effective way to train horses without long lining. If your bad knees prevent you from effectivly training this horse confidently, find a trainer.

Anyways, no second horse, get the horse on a lunge line. You can not effectivly correct a horse while free lunging. If a horse turns to kick at me while lunging they get a nice jerk on the lunge line, smacked on the rear with whip and a very loud NO. Kicking at you is not acceptable and it needs to be stopped pronto.