What do i do for him?

So my bfs birthday is coming up and i want to do something nice but i just dont no wat to do/get him. wev been dating for a lil over 4 months but he hasn't realy told me much. he like music,,, thats how we met,, in band class. thats like the only thing i actually no that he truly luvs to do. I no that ud probably say "o get him a book of music so he cn play that" but i want to get something realy special. (at my birthday he took me out to dinner and movies and gave me a beautiful necklace,, thats y i want it special) so any advice at all for wat i can do/get????? Pleez?