The keyword is, is I want. What does your SON want? It’s not about what you want. If he’s not interested in a “blast, a blow-out, a rocking good time”, then why on earth would you insist on it? His birthday, his decision.
You’d be surprised, though. Kids really do want parents to think they’re proud and starting to see them as adults.
IS there anything you could do/buy that’s an extra privilege? Clearly a car is out of order, but anything else?
He’s 18, so he likes music, right? Concert tickets (not that he’s taking you)?
The thing is, we don’t know what the kid wants from the OP, and, honestly, unless you know he doesn’t want a surprise, the fun of a birthday party is being surprised. So asking him directly is kinda out.
And, no, it’s not being selfish to want to give someone a big gift on a big birthday. It’s only selfish to insist on giving them the gift when you know they don’t want it. And I don’t think we know that yet.
Concerning the ideas in this thread, I like the concert one, and would extend it by adding other options. Anything you know your kid likes to do would work: Sports, movie, play, etc. Even just a bigger than usual gaming party where you buy him a game he wants and rent some fancy equipment to play it on. Or, even, as you suggest, paintball, if you know for sure that’s something he really likes. The surprise there is in telling him he gets to do it, but then you can also let him decide who to invite, just like it would be with giving him two tickets or creating the party your wife wants to give. They’re all good ideas.
Oh, and sticking a fiver in a card is only okay as a joke gift, unless you normally just give that sort of thing. People may disagree on how special an 18th birthday is, but they do tend to agree that it’s no less special than any other birthday.
Throw out a few options and ask him what he want’s to do.
Excellent points from everyone but Citizen 
He’s had his own SUV since he was 16 and we just got his Amateur Radio call sign on his tag last time he renewed. AR tags are free in GA - the only good thing Jimmy Carter did IMHO. 
He’s in on the deal - it’s his deal - and so a massive gaming, music and food fest it shall be. I only wish our pool was heated so they could swim as well!
We have all 3 gaming consoles, so there will be separate Wii, PS3, and Xbox rooms for everyone’s convenience.
How many pizzas do you think 30 teens can eat in 6 hours? :eek:
All of them.