Due to your boys age, we can probably rule out an abscessed tooth.
However, it is very possible he has something stuck in his mouth, a needle, pin, burr, etc
It is hurting a LOT if he will not let you look.
It also sounds like Tetanus (yes, dogs do get it!!)
It can be a result of a small wound your boy got months ago:
Please read the entire link.
In any event, he needs immediate veterinary care.I would have him see ASAP so you do not get stuck in the ER later
Please let me know how he is feeling. I will aways be here for both of you
I hope your boy will be ok.