What I would do to help his discomfort, is give him a baby aspirin (1 tablet per 15 pounds of weight, 2 times a day). Also, since there is no swelling at all I would just try to keep him off of it. But if he wants to move around let him, just no overexerting himself playing. If it does start to swelling you can ice it, 10 minutes at a time with at least a 1/2 hour in between.
Puppies are so delicate as you are well aware. That sometimes they bruise very easily. It is very possible that a good nights rest will help him a lot and he will use it a little better tomorrow. If he is a little better tomorrow, see if he improves the next day. If he is the same or worse tomorrow than you can have it checked at your regular vet.
If he has any other problems tonight, just reopen this question and let me know. It sounds as though you have done a very thorough exam and understand what you were looking for, so he has a very experienced nurse at his bedside. I tend to be overcautious and encourage people to go to the vet, because many get nervous and very upset because they do not know how to care for their puppies and kittens. Like I said if you need anything later on, just post me.
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Peace and happiness to you and your pet! ~ Christina
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