Excersize is very useful for getting the guts going, and one method I've used with great success is swimming therapy. I'm not sure how wise that is so soon after his op though.
I've usually let mine have a swim around in a shallow bath, one end just deep enough to take the weight off their legs, and held my hand under them as they do laps. If he's not too keen on that then you could just lower his back end into some warm water for a similar effect. Though as I said, unsure about this approach due to his stitches. You could just try getting him to run around the lounge or hall, depends how likely he is to wander!
Did the vet listen to his back end? It's possible to hear the disgestive system working, and from memory there should be roughly 2 distinct "waves" a minute. It's not impossible the op has slowed his digestion down, and the vet really should have looked into things a little more than "he might be constipated" in my opinion. I'd suggest getting him back to a vet tomorrow, and ask them to listen to his digestive system. If it's not running at around 2 cycles a minute, then an injection of Metachlopromide (**** spelt that wrong) would most likely be a good thing. It can help with hypermotility (gut going too fast, almost shuffling the contents back and forther rather than through) as well as hypomotility (slow down/ stasis). But it's not ideal to give to a piggy with a fully functioning intestine, so it's important to try and get the vet to check with a stethoscope before giving it.
This is of course if he's still seeming to have problems - Last Friday to now is quite a while for things to pep up for themselves (and equally, quite a while if they haven't). If he's eating, wandering around, pooing and being generally as he was pre op then be less concerned, although still worth a trip to the vet just to be safe.
Edit - Oops, missed the newer posts - Impaction is possible (surprised if the vet didn't pick up on that though?) and it's not too tricky to "unblock", if you're very careful and patient. A little KY, vaseline or even plain warm water will help ease the lump away.
Still worth seeing a vet as it could be the gut isn't working properly which has caused the impaction.