Hmmm, I have a 3 year old female who ALWAYS has her mouth open slightly. It looks like she's smiling or trying to pucker up for a kiss.
She was one of my first breeders and I panicked and took her to the vet. She was fine.
However, since this behavior is new for your boy and his lip is puffy (not split, cut, etc), I would imagine he ran into the side of his enclosure and is too embarrassed to admit it.
Really nothing you can do at this point but watch and wait. In all likelihood he will be fine if there are no other symptoms of discomfort or distress. But do listen out for signs of a respiratory illness as sometimes an open mouth can mean difficulty breathing which means he gets to go to the vet.