Before he was limping, did he play with other dogs? Was he running roughly with others? Is there any chance you might have heard him yelp?
When you touch his leg, does he wince or show that he's in pain? Does he pull it back? When Peanuts was young, she used to have this mannerism or raising her front paw while sitting or standing at a halt. I guess it was because she was a terrier mix.
It might just be a pulled muscle because sometimes play biting can do that. Jumping from the couch can also cause a limp leg (especially if he's small).
If it continues for more than a week, then something is up. Limping due to a pulled muscle only last for a couple of days if not hours.
