If the skin didn't stretch at all, he can take a larger size. Rule of thumb is the prey is wide around as the largest point of the snake, weekly, until they are up to a small rat size. Even then, males can stay with a 60-80 gram rat for the rest of their life.
It does sound like the onset of a nasty respiratory infection. Take him to a quality reptile vet, get a culture done, and get the anti biotics to nip this in the butt.
Here is a list of links that can help you find reptile vets in your area.
http://www.herpvetconnection.com/
http://www.anapsid.org/vets/#vetlist
http://www.arav.org/
I hope this helps