Well usually reactions occur right away. That is not to say that other reactions dont occur later. They probably shot him up with benedryl after to counteract the immune response he had to the vaccine. Ask your vet if your baby needs to be on benadryl for a bit. I've never had any reactions with my three, so I'm not that helpful in this regard.
I personally would be very careful about medications too.
Here is a link to Dr. Dodds - she is kind of the vaccine guru. I'd also check out the American Animal Hospital Association (AAHA) guidelines - not sure if they cover "what to do if" scenarios but at least it's a start!
W Jean Dodds Vaccine Protocol VACCINATION PROTOCOLS http://www.aahanet.org/PublicDocumen...s06Revised.pdf