My son started collecting Pokemon because the kids at school did. So I was like, well fuck if I'm paying all this money, we're going to learn to play, so we did. The end goal being MTG, which I've only recently gotten him into playing.
Depends how old your son is (we started Pokemon at 6, MTG at 9), but Pokemon is probably a better starter game both because every pack has a foil card (ohhh shinies) and the game is significantly easier to play. I made some MTG decks for us to play out of my boxes of common cards, picking out pretty basic creatures and spells, just because the game can be stupidly overly complicated these days if you're just starting out AND young.
Bonus - Pokemon packs/boxes are typically a little cheaper, but you get chances at pulling cards quite literally worth hundreds of dollars even from new sets. My son is quite happy knowing (and I'm sure bragging to his friends) that he has 2 cards each worth several hundred dollars. Cards dad keeps in a special tin for him so they stay in good condition.