Given that this is a catered event, I'd RSVP no for Ron and tell him why.
Otherwise, you and your wife *are* partially responsible for the wasted catering costs if Ron doesn't show up, because you know very well that he likely won't but told the hosts that he would. If the invitation had come to him directly, it would be his responsibility.
And even though Ron is an adult, I do think that him not showing up will still reflect badly on you (his parents) because of the circumstances. You sent back his RSVP, and you're the ones who will have to explain to the hosts that he has decided not to show up when you arrive at the event. They probably won't think well of him either, but he won't be there to deal with any consequences.
Parents are frequently considered responsible for the behaviour of their minor children. Ron is technically an adult, but it sounds like his position in the family is still very much like that of a child.
As an aside - have you actually talked to Ron about how things work now that he's out of high school? Or is this just based on three different sets of expectations (Ron, your wife, you) which are clashing. For example - tell Ron outright that he is not required to go to family invitations. He can say no, he doesn't want to go, but if he does say yes he is expected by the rules of etiquette and basic courtesy to show up. Or for things like dinner - telling him that if he expects to be considered part of the family for meals, he has to give advance notice about whether he will be there or not. If he isn't willing to do that, he's on his own for his meals.
I will say, though, that if you and your wife are giving him two different sets of expectations, he's going to go with the one that's easiest for him. So he'll flake out on invitations, and not come home even though you've cooked dinner for him, but unless you've got a united front and consequences for his behaviour, you've got a snowball's chance in eHell of changing that behaviour.