I am so sorry your boy is ill.
This is called myositis:
It sounds like he has dental issues. His Carnassial molars, specifically:
This accumulation of white blood cells at the site of an infection or necrotic material is referred to as pus or an abscess.
Owners often confuse this condition with an eye infection, insect bite, or puncture wound. They may consider it something that, if left alone, will heal on its own. The untreated abscess will, in fact, often spread to 1) the eye causing a very serious and potentially blinding infection or 2) other teeth causing them to be lost also. This is fairly painful for the animal, especially when eating. In dogs that stay outdoors or those with long hair, it may remain unnoticed for a long period of time.
For today he may have one mg per lb of benedryl. Feed him baby food with no onion powder. See his vet ASAP.
Please let me know how he is feeling. I hope he will be fine.
Sincerest best wishes,