A great big kiss and a hug (more if appropriate). Then take him someplace that you both know he loves but you don't, like a hocky game or a NASCAR race. No surprize parties unless its' all guys and there's beer. Make two things obvious: 1. You love him. 2. The gift is for Him.
Everyone knows how important it is to share common interests in a relationship. Tell your guy that you also have a birthday - in fact, you can prove it by wearing your birthday suit! One look and he'll want to celebrate more than once a year... Happy Birthday to all!
Hopefully you know your boyfriend long enough to know what he likes. Look at what he wears (not what you wish he would wear!) and find something similar but nicer. Guys also don't go looking for new cologne, but will wear on that you buy.
Boys usually know what they want, so if you want to know what to get him for his birthday you should just ask! If he doesn't answer or says "he doesn't care," you can get him something that co-relates to his hobbies. Or you can dress up in lingerie and give him a lap dance.