Hi and welcome to Just Answer,
Initially I need more information to be able to help you.
When did the diarrhea start?
Is there any jelly-like mucus or blood in it?
Does he go in one spot one many small spots?
What do you feed him? Canned or dry?
When did the rear leg stiffness start?
Is it worse when he first gets up and after exercise?
What breed is he?
How long has he gone without drinking?