If he has been constipated, it may result in a small fissure, or tear, of the sphincter (sp?) which whilst healing can cause a thickened edge or something that looks a bit like a pile/skin tag. If this is what it is, they do usually self-heal... but obviously you'd have to get it checked out for a proper diagnosis. I'm only guessing as a friends' daughter had this as a young bub.

Let us know how you go at the doc's. Good luck.