You need to know where exactly it is and if he is feeling an ache or pain. Use a scale. Ask if the pain is a like a person jabbed with a fist or thumb (ache) or like something is inside and pulling a tiny muscle (sharp pain). You can then determine if this is an inside bruise to the muscle from maybe sleeping the wrong way or a pulled muscle or pinched nerve and handle it appropriately. Both of these need hot and cold compresses and maybe massage to relax the tensed tissue. Aspirin may help.