I have a friend that is a "film buff" that is much more reckless with what he exposes his 5 year old son to (and his son seems somewhat normal so far). I'm talking like Chucky, Halloween, It, the works.
So I don't think it is possible to do significant damage if you are at all responsive to what the kid thinks is too scary.
I personally think the Alien series is fantastic and a great place to start.
I would go: Prometheus, Alien, Alien 2, Alien 3, Covenant if you want to really go all in on this holiday break.
Not a lot of overt/uncomfortable sex scenes and lots of gore/sci fi action. Plus some good down time to talk if need be.