Marvel Splendor is a good one, if he is into card games Marvel Champions is another good one. 5 Minute Marvel is another popular game.
The Unmatched Game System is on a lot of top lists recently and they have a set of Marvel-themed sets, with more planned I believe:
Personally, I love the game. I've had good experiences with gamers and non-gamers alike. I was originally concerned that the Marvel expansions would be subpar, relying on the IP, but, honestly, they have some of my favourite character mechanics.
Marvel does a bunch of licensing for pre existing games to release marvel reskins, so if you know a few games he likes, maybe see if any of those exist.
If going away from marvel isn't a deal breaker, king of tokyo is a great game that has a colorful, marvely, comic book aesthetic. It's also a great entry level game for a family that may not be seasoned board gamers, but with enough interaction and randomness that you won't get bored once you get the gist, and nobody can really outskill anyone else.
There is a counterpart game, king of new york, that gets even closer to the marvel theme but the community mostly agrees that Tokyo is the better game because it's more simple and streamlined vs new York which adds a tad more complexity. NY does have it's fans though and your step dad might appreciate it more if he's a huge marvel guy
In my case, I enjoy marvel board games that bring out the characters' style a bit more than a skin on a good board game(already have plenty of good games). The already suggested Marvel Champions is a great coop where you play a single character trying to defeat the selected villain. I'll also put forth Marvel Legendary, it's a coop deckbuilder. If he has some favorite Marvel characters both of those games have oodles of expansions that you can mix and match.
Depending on experience level Marvel United (simpler) and Marvel Champions (more complex but more thematic) are some very popular games. BGG also has a category that lists a bunch of other options.
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