I remember being that age... and yes, at that age and like your son prefers... a "party" is not really wished for, nor a big "family" thing. Ugh, is the word at this age.
Perhaps, instead, you can just do something private, and just only your family. Besides high school graduation/party (which is a milestone and a rite of passage anyway), this is usually where the "child" is celebrated for becoming an "adult."
I would say, that if your son REALLY prefers "not" to have a big-deal party, then respect his wishes. At this age, NO child likes cutesy-fun parties. For me, when I was that age... a "party" even if for myself, was just kind of unbearable and more of an "appointment" than "fun." I much preferred to be just with my few best friends, and have a private fun time together that way.
Does his other friends have parties like this, for turning 18??? Is this something that his circle of friends does? Maybe that is something you could think about as well. If this is not the "norm" for his friends or his circle of peers...then maybe he might feel "odd" if his Mom does this sort of party for him, and him being 18 and not a "kiddie." Then again.. .is your son the type that "likes" attention, or not?
The thing is, you CAN "celebrate" his turning 18... but it does not have to be a "party" and publicly celebrating it. It can just be, instead, something low-key, between the both of you, or with Hubby and his siblings... or, just something private. After-all, if he does not want a party.... and you make one, then the party would not be "for" him, right?
Why don't you just get him a real special gift/memento...something that is truly meaningful and that he will treasure forever.... instead?
I would go according to your son... and what he wants. After-all, he is the "adult" now, right? And to me, he doesn't seem to really "want" a public big celebration anyway, nor anything that is a big deal.
If my Mom made me a big deal party, after my explaining that I didn't want one, and being 18... well, I would feel mixed feelings about it. It's best to talk to him.... and then respect and listen to what HIS wishes are... perhaps, HE would like to celebrate the occasion in another way.... it's HIS special time anyway, and he already seems quite mature and independent...having his own work and work friends and friends.
All the best,
Susan