We took him to the emergency vet, and they kept him a few days and took blood samples and were able to open his jaw to see if he had a tooth abscess
and they took xrays also. He was given antibiotics and steroids. They said they couldn't find any cause or problem, and was very frustrating for us because not only was that visit and procedures over $1,000 the swelling is now returning. The swelling did go down while he was on steroids. He is eating and running about normally.