1. Does he have friends that will play? A lot of wargaming is just garage folks. That requires a group of people you can play with regularly. Kinda sucks to get into a game and you have no one to play with.
2. Go to your local game stores that do wargaming and see what the scene is. Plural stores is important here. The store with the biggest scene might actually be the worst to play at or host the dumbest players. In my experience, there are shops that have lots of models and accessories with huge play areas and there are shops with great scenes geared for all players. Sometimes those intersect; often they do not. You’re looking for a friendly place willing to cater to beginners. Good starter scenes have lots of people willing to share armies so you can see what’s going on.
3. How much do you want to spent right now? Kill Team, Combat Patrol, full 40k in that order is probably cheapest. AoS doesn’t have the cheaper options yet AFAIK officially but there are some smaller skirmish options.
4. Marvel Crisis Protocol and Star Wars several things might be better options based on the first three points depending on your wallet and local scene.

However, in my experience, once you start one of these things you usually dabble in lots especially if you’re at a good store.

There’s usually one or two bundles a year that give you plenty to read. I have so much content from the last few years.

There are so many entry points that if you don’t have a very specific desire it doesn’t matter a ton where you start.

The Humble Bundle is a solid deal. The Tome of Fire trilogy is a big Salamanders line. The rest is pretty standard current space marines stuff. If you’re looking for backstory, specifically Dark Angels, tracing their path through the Horus Heresy novels is a good idea. If you’re looking for non-marines starters, Eisenhorn and Ciaphis Cain are good starting points. I am personally fond of the Vaults of Terra trilogy (have not been able to get into the sequel yet). There’s only a few Necron novels so if that’s your thing it’s pretty easy to start. I don’t know a ton about the other races and their fiction.

I really appreciate the Emperor callouts. In 40k he’s way too removed to be anything more than a swear. In 30k so far I’ve only directly seen him in one of the short stories. I don’t have an opinion on him so much as I have opinions on how people are executing his vision. Maybe I’m being too sentimental about the callback when chaos steals all the primarchs somewhere early in the first HH trilogy.