Get his favorite wine
decorate w/little white lights
frame a picture of you both from when you were younger (like a favorite
fun party picture or something)
make a list of 40 reason why he's special to you and why you love him
hide presents at the neighbors house that you trust or a friend's house, a
relative's house etc. (keep inexpensive ones in your trunk if won't look
there.)
A happy birthday sign out front
A CD of his favorite songs that you put toghether
A party w/some of his friends. You can hold it at a restaurant bar so it can be low-key and casual for an hour or two.
If you live in the town where he grew up in, go around w/a digital camera and take pictures of all of his old haunts: the house he grew up in, his high school, grade school, place hhe used to "hang out" etc.
tickets to his favorite sporting event
Have a customized wine label made & put it on his favorite wine.
Something engraved for his desk.
Have fun!