You need to read over www.guinealynx.info/postop.html It has tips for neuters (flushing the incision daily). You need to be weighing him daily, watching for signs of pain, hand feeding if he is not eating.
I would contact your vet if you are worried about the swelling.
A hematoma can develop pretty soon after surgery. They usually go down in size in a few days. An abscess, however, usually takes time to get big. If you notice a swelling in the neuter area a week or more after, that's most likely an abscess. Also you can often smell pus and maybe even see some oozing out of the site. Abscesses are pretty common after neutering, so I always give at least a week, usually ten days, of an antibiotic. Bactrim works best for abscesses.