I've never run into this problem myself so I'm not sure I want to suggest how to check...you'd somehow have to figure out how to get his mouth open and look inside...Do a google search for ball python impaction and you'll probably find some info on it.
It could also be that he's having a problem realigning his jaws. It usually only takes a few "yawns" to get it straight but maybe he's having problems? Does his mouth look like it's closing normally?