I would just keep him very very quiet for the rest of the night, and see what he is like in the morning. Just like us, if he has pulled a muscle then tomorrow morning it's going to feel stiff and sore. But as he walks around then if there is any sign of that stiffness it should disappear slightly as he warms the muscle up. If it continues to get worse and worse than I think you need to see a vet.
Basically, follow your instincts.
I'm thinking he pulled it whilst playing and will be ok with rest.