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.
In the case of a carnassial tooth, the abscess builds up around the affected root just under the skin below and in front of the eye. The swelling may reach the size of a golf ball. In the case of an abscess, the white blood cells and chemicals that are released have the ability to dissolve the body's tissue. The weakest portion of the body in this case is the skin, so a small pore soon opens from which pus (or a pink-tinged fluid) will drain. Left alone, this opening will occasionally close but then reopen later as more material accumulates.
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,