I found a similar question & answer like this from a site i was recently browsing, read the Q & A, and a little bit detail would have helped, like the species, if he was feed live or has sustained any damage to that area recently . But with minimum detail here is what i dug up, (Go to the site for the question)
"Swellings can be a number of things, most of which should be checked out by a vet straight away. It could be a cyst or abscess, celulitis, or cancerous tumor.
Cysts and abscesses aren't usually a big problem. Cysts can contain air, fluids or a semi-solid material and abscesses contain pus. They wont do any severe harm to your animal but may cause pain and so should be removed by a vet.
Celulitis is caused when the skin is broken by either cuts, abrasions or burns etc. And bacteria enters. The bacteria multiplies rapidly and causes the area to become inflamed and swollen.
A cancerous tumor, which is the most severe of all the above, must, if possible be removed, as soon as it can. Especially being so close to the head, cancer can spread and may enter the brain resulting in certain death for your animal .
pets question and answers,www.5d2d.com Although it may not be any of these, it is a good idea to have him checked out by a vet just to be safe. "
I think he just has some type of fungus under him maing it swell tak him to the vet and get it checked
