He is OK!
The lump is inflammatory cells, not cancerous, not even a cyst. It could have been caused by any number of things like trauma, bug bite, infected scratch. However it happened I am just so relieved it's not anything more serious. So he starts his 10 days of antibiotics tomorrow and we'll just keep an eye on the lump to make sure it doesn't grow or start to bother him.
Thanks for the comforting posts everyone, I really appreciate it.