
It could be a number of things but the vet is the best place to take him for advise.
What age is he? this would help at least break down the likeness things.
It could be a simple thing like a strain which is going to take a bit of time and rest to heal or it could be something more serious it cruciate ligament, it could be arthritis if he is old.
You haven't gave much info for anyone to go on :-)