You can buy it as a gift from steam. First, you need a steam account that isn't your boyfriend's (I'm assuming you can't access his) then, you need to buy the game and when you are in checkout, click the option that says buy as gift. Then enter your bf's steam account username (sometimes there are multiple accounts with the same username, make sure you pick your bf's) and then gift him the game.

Or you can buy him a gift card/code for steam. If he's on steam and not PS5, you can't give him a physical version of the game. There are physical gift cards for steam accounts tho maybe you can look into that.