OK, firstly, I'll make it clear that I'm a 22 year old with absolutely no intention of ever having kids. I'm not a psychologist either. Take from that what you will.
Nothing wrong with a lot of movies as far as I'm concerned, provided they're the right kind. The Mummy films are really just a reincarnation of the old "saturday matinee" fare, like Errol Flynn films. Star Wars, damn near all Superhero stories (ESPECIALLY Spider-Man and Superman) and most of the early westerns are also similar. That is, clear good guy/bad guy dichotomy, no 'real' violence to speak of, and eventual heroic victory. I first saw Star Wars when I was 4, and absolutely loved it. ET, on the other hand, scared the crud out of me a year later. My friends were all similar, most kids I know, of both genders, love those sorts of stories. And we all turned out fine. Well as fine as gamers ever are!
Superheroes actually deserve a special mention. Show me a single young boy who doesn't love superheroes? Superman and Spider-Man are great examples. I'd probably steer clear of X-Men and 'purer' Batman material, given the moral ambiguity that's omnipresent in them (or maybe that's my Batman fanboyism coming to the fore.) Let the kid watch stuff like Justice League though, 99% of any objectionable material's just gonna go over his head.
Make sure he understands consequences though. No matter what you do, he IS going to emulate the screen action, and the sooner he learns to avoid injuring himself or others, the better. Don't shy away from disciplining him if he messes around either. Lock the real swords up good and proper, and move them somewhere else, I can tell you that all kids from age 2 to 6 have the equivalent of several rogue levels when it comes to finding anything REMOTELY dangerous. (One good friend of mine has a 2 year old who's just learned to open the supposedly kid-safe latches on their cupboards).
Let him be a kid. Heck, I'd be more worried about him watching The Simpsons than The Mummy Returns.