I agree with Smooth Liar, I was reading the other answers and feeling decidedly uneasy about leaving this "kid" alone, he's really a bit old for teen-angst, he should be through that by now and in full time work as a productive member of society, not sulking around the house refusing to have a birthday.
Something's wrong, don't leave him alone, see if having a small get together is acceptable once it's under-way, not like having 30 people over, I mean taking him and yourselves to a place you know he likes, and seeing if he has fun once he's there. If he doesn't, you may need to consider him seeing his doctor to discuss things.
People are not suppose to sit around on their own refusing to interact. A healthy person, even when sad, will try to fit in and not upset their family. When they start disregarding you in that fashion, it tells me it's moved from "a bit down" to Something Actually Wrong.