I like giving people gifts, but I honestly have no idea what I could give my boyfriend. Band merch is way too expensive in South Africa so I can't buy him anything like that. What kind of things could I make or buy for him? He mainly listens to black, death and thrash metal and he plays drums and electric guitar. Any ideas?

A guitar strap maybe with the logos of some of the bands he likes painted on.

Custom guitar picks! Just use old credit cards and trace a guitar pick onto the hard card material then cut out and sand the edges with nail file. You can also paint them if you want to make them more unique.
You could also cut a normal pick in half and make "best friend" necklaces. Or you could embroider a guitar strap for him with his favorite lyrics.

Even a custom painted box to keep picks and strings in would probably be appreciated.

As felicity mentioned with the custom picks they now make a punch that cuts guitar picks out of credit cards and thick plastic.

Those punches are ridiculously overpriced here. I work part time so it's 3 months of my pay to buy it so I'd never be able to afford one

Ive had many metal-head boy friends. lol & heres a list of things they've stolen from me that they liked. LOL

Bracelets (any simple braided/weaved bracelets in colors they liked). Handmade Patches, buttons, stickers of bands/things they like. I also made a lot of meshed/slashed t-shirts for them ...made from t-shirts they already had (just takes scissors). & you could copy any band-shirt on a stencil any make them yourself on a tshirt/hoodie/or pillow! They usually like dark artwork too, maybe paint them something? And my friends have usually taken anything, literally anything, with skulls on it. XD Paint skulls on his drumsticks! oh, & men always like food, and their favorite drinks.

Guitar pick bracelet? Bake him a cake for his birthday?