Loosing fur on stomach

Sweetie has a  largish bare patch on his stomach just a little way up from his back legs.  I haven't noticed that he has been over-grooming recently, and I have no idea what else could have caused this bare patch.  In all Sweetie's previous vet appointments the vet said his health was excellent so I'm not sure if this is something to be concerned about or not.  Any ideas?

Is the patch red or rashy in appearance? Exactly how large is it? Overgrooming could be a cause. Allergies or irritation. Flea or ticks. There's a lot of different scenarios.

It looks like normal skin and it's roughly about one and a half inches long and an inch wide.  Sorry but I'm not able to post any pictures at this time.