Just got back from the vet.....very good news!
It seems this lipoma became infected somehow and that is why it suddenly became so large. There was no wound on it so vet thinks it may have been an ingrown hair or tiny scratch. They froze the area, lanced it and drained all the junk out. He's all bandaged up because the wound is a bit open and bleeding but other than 7 days of antibiotics and topical cream, we are all good!
I feel stupid for thinking the worst