Okay, it just sounds like he's cut himself on something and it's becoming a bit infected, because all pus is just white blood cells protecting from infection. Don't let people stress you out. I'd go down to a produce shop, get a spray can of cetrigen, call the vet, keep the wound clean, and just watch it. Cetrigen is just a purple spray that will help dry out the wound and keep flies, etc away from it. I hope all turns out well.