So the thing with modern buying of movies, is that these movies are tied to the account you bought them for, so are stuck with rights management. There is no simple way to just buy a movie and copy it to a drive unless you buy the DVD/BluRay and then use ripping software to copy the movie to the disk. A lot of people do this with their home collections though. I have all of my stuff on a drive in MP4 or AVI (Divx) format so I can play them anywhere. It will require a bunch of time though, get the disk, copy the disk, and you probably want to convert the disk to another format to save space, especially for BluRays.
You can download and save movies with Amazon, Netflix and others, but you will need a device signed into that account to play them back. So an option may be to buy a tablet, download the things to it, then he would need to authenticate the account where he is, and then he can view the downloaded media. Benefit there is that the media is on the device, so you just need a weak/short time point of internet access to logon. If he has some sort of internet access, even slower or temporary, that would work. I do this on my long rides with family, download some stuff at home, then it works with no internet for a while. At some point, you will need to logon again to re-activate the shows/movies to the account, not sure how long that is but I'm pretty sure only every few days.