Many pups feel a "bit under the weather" after a round of vaccines. I'd wait something like 48 hours and if there's no improvement, call the vet to see if something else is needed. I don't think a 2nd visit is warranted unless he takes a turn for the worse. Things to watch for are fluid intake and bowel movements. If he stops drinking, has diarrhea, or has a fever. You can check for fluids by pinching the coat and gently then drawing it up. Then let go. If it rebounds, he's getting enough fluids. If it stays pinched, he needs fluids and possibly immediate attention.