I think at his age they say go to doc if it persists more than 24 hours. I'd possibly take him back again just to check him over. It can really hit them hard though. I just had a tummy bug last week... not a vomiting one thank goodness - had that one 2 months ago - anyway, took me about 4 days to stop sitting on the toilet and close to a week all up to feel pretty good again and it wasn't a bad bug really, more annoying. I hope he feels better really soon the poor little guy.