Rich,
I’m not a fan of the reboot Trek movies, but if you want action, adventure, aliens, explosions and characters that are loosely based on and are superficial shells of the loveable ones you’ve spent a lifetime watching, give those a go.
Of the *real* Trek movies, Star Trek II: Wrath of Khan is your best bet. The only one to avoid — and I don’t say this lightly — is Star Trek V: The Movie We Do Not Speak Of.
All that said, the best Trek has been on TV. I’d start with The Doomsday Machine. If his interest is piqued, expect years — nay, decades — of nerdery ahead.
Joe