I’d have to say that at 15, anything would be completely mindboggling to me. But then, I didn’t have anything remotely close to a girlfriend until I was 18. Is your bf used to having girlfriends or is this all new to you?
One thing I’d worry about is overwhelming his present. Or worse, what if he doesn’t get you anything? But then, this doesn’t matter if he doesn’t know you exist.
I would not get him anything very big unless you know for sure he is going to make a dramatic gesture in return. Otherwise something small. Chocolates seem too much like a Mom or Dad gift. Flowers…. eh. He’s a guy. Is he a sensitive soul? An artist? Then a poem or a story or even a fancy homemade card would be nice.
Also it could be a book that means a lot to you. Maybe poetry.
Similarly, something that gets you off on your own together. Tickets to a show you both like. Or make a picnic and take him out to a park or the country or some place it’s just the two of you. If it’s cold outside, then it’s better to go inside, but not necessary. My first kiss took place outdoors in the snow. But I’m weird and I don’t recommend that. Her lips were like dead worms, I’m afraid. So, ixnay the old-cay.
Anyway, I think that’s what I would have liked best at that age—something personal, or a date somewhere where we could be just the two of us and I could dream of kissing. Of course, maybe kissing is no big deal to you, but I had never been kissed when I was 15. Sorry if my advice is so old-fashioned and irrelevant to be useless. Just an old man dreaming of a different past.