what should I buy?

Maybe a giftcard to his favorite store or store you know he could find somethign he likes. Or maybe a gift card to a resturaunt?

the biggest present to him is the presense of you near him on his birthday.

Hi there! Well, I think you should tell us about him first before we can help you pick out a gift for him. What is he like, what are the things that he likes, his interests and so on. But some safe suggestions are gift certificates to his favorite store or restaurant, game or concert tickets, a shirt, a cd of his favorite artist or something that he can add to his collection if he collects anything.