Without a picture and going by your description, I would guess it is a umbilical hernia. You should have your vet check it out to make sure though. Pyrate had one his entire life. The vet said it had filled in with fatty tissue so there was no chance of any problems. It never bothered him. I hope this is the case with your boy.
Snake Vom Eselspfad RN CGC (AKA Raina) 3/5/08
Pyrate CGC 4/1/03-5/16/12 RIP
Too many to list waiting at the bridge but remembering them all