One of the few things the Feds have done right for gun owners is to pass a law EXEMPTING travelers from the firearms laws of the areas they pass through.
If he is doing all the actions CONSISTENT with being a traveler--i.e. traveling on roadways, staying overnight in a motel, etc. The guns are stored inaccessably (trunk, locked up, unloaded, etc.), he is legal provided the guns are legal where he is coming from and going to. If he decides to stop along the way, go to a movie, visit relatives, sightsee, etc. he then looses that protection while he is there--so he must then follow any local regulations.
That is why you can drive through very anti-gun areas without any legal problems. Some LEO's don't realize the protections, and have been known to on occasion cause problems. Best answer if he is stopped, and asked "do you have any guns" is to ask the officer POLITELY "why are you asking me that question?".