Ask him what he’s into. Does he really like Pokemon or Final Fantasy characters? The play the Pokemon or Final Fantasy TCG. Does he want to try Magic? Play Magic.
Almost all TCGs offer some kind of 2-player starter kit. Along with the tutorial booklet and other materials, the decks are also a good way to introduce new players because they tend to feature simpler cards with a consistent theme and a limited number of keywords and other fiddly mechanics. A lot of game stores will also hold TCG events for beginners, or offer free demos where they walk you through the game (sometimes even with free starter decks). If you can find a game store with nice staff, it’s a great way to try out a couple different games.
LCGs are definitely a good option, especially if he really wants to get into deckbuilding. It can be a bit overwhelming to get the whole set all at once, but it’s really nice having the freedom to build and experiment with any deck you want, and not have to worry about browsing ebay auctions for rare cards. Fantasy Flight has a few different LCGs currently running, attached to various brands
Most of the digital card game players I know are people who started out playing traditional TCGs, so I’m not sure how good it would be as an introduction. Personally, I’m a fan of Eternal, which is mechanically closer to MTG and is much less stingy than most F2P TCGs (more cards in each pack, and you earn card packs faster without having to grind for hours and hours). DM me for a referral code if you decide to try it, it gives you with a few extra card packs but only if you use the code when you create an account.
There’s also a lot of “Deckbuilding” card games available right now, which are nice options if you want something that emulates the feel of a draft. Ascension is quite easy for new players to learn, works with 2-4 players, and is mostly an indirect competition. Star Realms is more focused on direct conflict, although it loses a bit of the draft feel when you only have 2 people playing. Hero Realms is a fantasy-themed version of the same game, and also features a cool Co-Op campaign and some cool all-vs-1 “boss decks”. Ascension and Star Realms are both fairly established, and have lots of different expansions that you can mix-and-match (some of the expansions are also playable as stand-alone games).