our suggestion: go homemade! it's been tested by yours truly and confirmed to work. it'll save you a few bucks. and it makes this pseudo-holiday more fun. and cute.
channel second grade, get creative and make him a card. you may find yourself having a little too much fun with this one.
or lastly write (or type) him a list: 101 reasons i love you, 10 reasons you make me smile, etc. include the serious stuff but don't forget the funny stuff or the those insider jokes the two of you have.