You might want to check in his mouth for anything stuck between or behind the teeth, a broken tooth, a retained puppy tooth or a cut. Any of those can cause some mouth pain.
In addition there is a problem called myositis which as many potential causes which makes it uncomfortable to open the jaw. You can read about this here
http://www.marvistavet.org/html/body_masticatory_myositis.html
Tick carried diseases can also cause muscle and joint pain and discomfort.
A neck injury can cause facial area pain too.
If you can't find anything obvious and the pain keeps up I'd have your vet check him over.
Hope this helps you!
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