My kid is finally going to be and adult, but I don't want to get him a present for his 18th birthday
Really? That's callous. Why don't you want to get your son a birthday present?
It is his birthday, that does not mean I should buy him a present. It is not my problem that he has gone this far in life without learning how to take care of himself.
You should give your child a gift on his birthday. Not giving your son a gift will make it appear that you don't care about him. Instead, you should give him something sentimental.
I don't want to give him a damn fucking gift because he's an asshole.
That's no way to talk about your own child! Don't you want to make your son feel loved? Giving him even a very small gift will reassured him that you love him.