This bitch wants money now? He donated the sperm 18 years ago! Now as for the other particulars, he A.) signed the birth certificate, B.) has sent cash + gifts + signed birthday cards as "Dad".
If you want to be involved in the kids life, and you want the kid to call you dad, you should be expected to pay child support. Obviously this doesn't apply to sperm donors who remain anonymous. And I don't think this guy should pay anything either, since it was 18 years ago.