Was there any recent injury's?
Check the paw over to make sure there are not cuts, scrapes, bruises or anything lodged in the paw is there anything there?
I am concerned with this injury. Fractures are very common and are easily mistaken for strains. Where a strain or a torn muscle will gradually improve and fracture will cause continued pain and discomfort. He may still be able to place weight on a fracture and allow you to touch it with little pain, so this is a very real possibility. I would advise you to have it looked at.
It is hard to distinguish a fracture or dislocated hip from a strain without examination so it is always advised to have any leg or hip injury's examined.
Discontinue exercises until it has been examined. I would recommend keeping him off the leg as much as possible for at least a week depending on the injury. Keep in mind fractures that are left untreated can cause problems later on with arthritis.
You can also speak with your vet about getting him a pain reliever or about picking up some pain relievers from your local pet store to alleviate his discomfort.