anyone know of anything that can improve a cat's immune system?

My kitty (age 8) broke his leg in September, had it set with a plate, and then came down with a bone infection last month, had the plate taken out, started healing just fine, until... he started limping again this week - we took him to the vet's again and he told us that he re-broke the leg because of the infection. It's not treatable with antibiotics, for some reason, and we are just hoping for a minor miracle that his own immune system can repair it. He's having surgery again tomorrow, to set the bone with an outside fixator.

He is otherwise excellent health. Thanks, and I'll keep you posted.