Is she declawed?? If she is, she may very well have arthritis in her paws.

Picking the food up at night is a good idea. Also put away any trash cans as they WILL go looking.

Is there a "light" formula of the I/D you're feeding him now? If so that might be another compromise. It's very hard to feed special food to one cat when you've got multiples. I know, I've got 5!!

Good luck!