Check the mouth for a broken tooth.
Last year I had one who broke her back (carnassal) tooth in the spring. Wasn't giving her any trouble so we left it alone. This past May, a year after the tooth broke, I noticed a minor swelling under her eye. Figured it was a bug bite. Within 48hrs it doubled in size, so we obviously had to take her to the Vet ... came to find out that the tooth was badly infected, and the problem was that the root of that back tooth was very long, reaching almost to the eye. Which was why the swelling had appeared so far up.
She had to be treated with Baytril for the infection & that tooth needed to be pulled. She's normally a hardy dog but she needed antibiotics before & after the tooth came out
It's a very big deal when the swelling is in that area. Whether it is or isn't the tooth, the infection could easily spread to the eye or the brain ... Some things I like to treat on my own with fish mox, this would not be one of those instances. This dog urgently needs to see a Vet. Wish you luck with him & please keep us posted.
::back into lurking now, lol::