Did he have dissolveable stitches or ones that needed to be removed? I ask this because Molly had something similar after surgery, it was a piece of the suture making its way to the surface. It resembled a nipple. It popped on its own like a big pimple and I cleaned it up. A hard piece of something came out and it was healed in a couple of days. This was about a month after or longer that she had the incision. Maybe even 3 months, can't remember! My vet said it was a piece of the stitches that did not dissolve. Since then we prefer ones (stitches) that need to be removed. I just clip them off myself.