Thanks for coming here to ask! You're really being awesome in taking such an active interest like this.
There's 3 main kinds of gift that I would, myself, like as a Magic player:
#1 - Learn to Play, and Challenge him to a game.I'm a romantic at heart, and a family-orientated guy, so when my wife says she wants to have a game with me of Magic, my eyes light up. I love it when the people important in my life take an interest in what I do. If he's a bit older (over 20 years old), then this is probably what he'd like most. If he's younger, the it depends on how serious your relationship is (if it's pretty recent, he might still want this to be "his" time). But yeah, nothing is quite as sexy as my love showing me their mad skills and intellect. You can learn pretty quickly by playing Magic: Online, and then buy a deck, make some adjustments, and take him to town, likely for under $70 US, and give him the deck afterwards, but it'll take some time to learn that you might not have.
#2 - Magic ParaphernaliaHey, maybe you don't want to play, or it's his thing that you don't want to step over. In this case, go with what AzureShade said. A play mat, dice,
t-shirt, tokens, store credit, Magic-Online credit, or other Magic-related stuff is always appreciated and really sweet. My wife once gave me a small box with flower decals that I used to store my deck and dice for Draft nights at the store or with friends. Yes, this resulted in endless puns about me or other people "play with my wife's box", but we all had a good laugh about that, and in the end people really respected and appreciated that my wife cared about my hobby to give that to me to use, so it was great.
#3 - Booster Packs from the newest set, or a pre-made Commander deckThere's a problem with Booster Packs... they have a "best before" date. After 2 years from the time of original sale of a set, the contents of most packs are nearly worthless. There are exceptions (such as
Jace, the Mind Sculptor), but they're pretty few and far between. Packs are great for getting you access to the latest and greatest stuff, or for saving and using as part of a draft with friends, but they're so ethereal in how long they retain relevance. If he even gets any good cards from them, he will forget that it was you that got it for him. Instead, I'd recommend getting a Commander deck. Commander is a format where most (if not all) of the major cards in them retain their worth and value for a very long time, which is one of the reasons so many people play that format of the game (unlike the format where you can only use cards printed within the last 2 years). If you buy 2 of these decks, this is another way that you can get yourself into the game along with him.
Hope this helps!