Please do not ask the hosts to contact Ron personally as suggested above - they have enough to do in organising their own event, without being dragged into your family problems. I really don't think that would go down well with them at all (I do note that the OP has never said that he would do this).
With regards Ron, I think you need to have a serious talk to him about the consequences of accepting. If he goes ahead and does bail at the last minute, he absolutely needs to feel some consequences of this. I like the idea of the apology letter and perhaps some financial "hurt".
However, I do think that there is a massive difference between a young man of 18 choosing to miss a few family dinners at the last minute (yes, even when his father has cooked something specially) and him failing to turn up to a formal catered event hosted by a family he presumably likes and respects. Remember what it was like being a teenager? I'm sure we all let our OWN parents down on occasion (especially if one parent is supporting those actions as in the OP) but there is a huge gap between doing that and letting down someone ELSE's parents.
I think the posts here are a bit harsh personally. I don't think being a bit flakely at 18 towards your own parents (whom you really don't want to socialise much with at that age) is really going to affect his marriage prospects or career in the future.
I don't think the issue is about socialising with your parents, it's about respect. When I was 18 I missed loads of family dinners to hang out with my friends, but my mother always got a phone call about it or knew in advance. That was pre-mobile, so now it's even easier to call or even text and say "Hey, something's come up, won't be in for dinner." To not show even this really basic
level of courtesy towards your own parents does reflect poorly on a young man's character with many people.
FWIW I asked my DH, who works almost exclusively with other men in a very "male" environment, what he would think of a young man like this and the first words out of his mouth were "Well, he's obviously not very reliable." Obviously that's an 'interesting assumption', but it's probably a typical one for older males. Getting tagged as 'not very reliable' in a work environment like that really will impact on your future. Obviously if he IS a reliable young man other than this one thing that will show in time, but he is starting out on a lower footing than others may be.
You may think this is typical teenage behaviour, but it really genuinely creates a very bad impression with many people, and it's foolish for a young man to do that when it is such an easy thing to avoid. I will also say that I've known many young adults who've done this kind of thing, but they've done it ONCE before their parents have had a come-to-deity meeting with them and they've learnt a quick lesson in courtesy.