Something personal he would enjoy... he's turning 21... artist who enjoys Adult Swim...
Really, when it's from someone you care a great deal about, usually the best gift costs little to nothing. Try something that is meaningful, requires effort, but not necessarily requires you to spend money.
For example, a few years ago I had written a list for my fiance' (At the time... Didn't end up working out some time later.

) which was "101 Reasons Why I Love You". It may seem mushy, corny, or whatever, but if it is something that shows how much you care about them, how well you know them, and how much effort you put into the gift, it won't fail. I guarantee it.
Here's a few suggestions:
- Why not do a little searching on the internet and find out what song was #1 on each of his birthdays, then burn a CD of all 21 on them? Even if they aren't songs he enjoys, they'll all mean something special.
- Maybe make some kind of collage with pictures of each of you through the years? Start with toddlers and work your way up to when you became a couple and end it on his 21st birthday with a picture you take together when you give it to him and add it in that day?
- Set aside a day and devote it to him. Give him 21 little presents throughout the day (Make him breakfast, give him little candies, take him out to dinner, maybe a massage

) and with each gift give him a little envelope with a number on it (1 - 21) and tell him he can't open them until he gets all 21. At the end of the evening after your final gift, once you present him with the last envelope, have him open them all and within each envelope have a line to a poem you composed.
- Buy him 21 birthday cards and sign them all and work with friends and family members to have each of them give him one of them throughout the course of the day.
Those are just a few suggestions, but you can get the general idea. Good luck and I hope this helped!