He IS on antibiotics anyway, right?
Suture reactions are really not that uncommon...even in people. They use lighter suture material intentionally so that it will dissolve in a timely manner and not need to be removed...but it doesn't always work out the way it's planned. I had one dog that reacted to suture material on an aural hematoma...and it was really, in a way, lucky that she had, because when she was spayed months later, at a different clinic...I could tell them..you cannot use vicryl on this dog... make sure you know what type of suture was used...because if you ever have to go to an alternate vet for stitches..that information is important.