I don't mean to scare you, but he needs to be on antibiotics right away. Incision abscesses are very serious and should never be considered "normal". Metacam would also be a good idea, as he's probably pulling at those stitches because of pain/infection.
This is not a garden variety abscess, when sutures are involved, they can pull the infection all the way back down to the inner wall...that's one infection you do NOT want. It can become life threatening very quickly!
Again, not trying to scare you...but if it were me, I'd take it very seriously. I would have him back at the vet, on antibiotics and metacam. Your vet might need to go back in and surgically clear out the infection before it gets worse. :sad3: