Wylie had a growth once on his chin and it really freaked me out. It was like a hard bubble. The vet wasn't too worried about it at all. He said it could be a little wart or cyst and may easily go down on it's own. If it didn't, only then would he think about removing it. Sure enough, within 3 weeks or so - gone!
It doesn't seem sore or anything does it? The basic signs of infection are redness, pain, heat, and swelling - so if all of those are present, then I'd take him in....otherwise, maybe keep an eye on it for a bit?