You don't say how old your son is, but he's a father and is about to be married so is clearly a grown up, not a little boy anymore. I agree that apologies are warranted, but from both sides not just yours.
I can understand that you may have been upset when they talked of marrying abroad, but unless at the time you'd said you wouldn't go to the wedding if it was abroad, to now say they're only getting married here because of you is ridiculous. Perhaps you have, unintentionally put pressure on them with their wedding plans but withdrawing the wedding invitation is a deliberate act.
The son we are now estranged from married abroad because she'd fallen out with her parents and didn't want them at the wedding. At that time when I foolishly believed we had a good relationship, I too suggested that she re think her decision, getting married without her parents was a huge step and she may later regret it. I see nothing inappropriate in what you said to your son. She has since reconciled with her parents and it is now us who are out in the cold and have been for 3 years.
We are all guilty at times of being as Elrel has said sharp or tactless but uninviting ones own parents from your wedding is IMO is worse than not inviting them in the first place. Having your attempt to try and talk about the situation completely ignored is jut making the situation worse and is very wrong of your son. It's amasing how deafening silence can be.
I too believe that if they had not married we would never have lost our son. My DH, our family and friends lay the blame at her door but he is responsible for his own actions/inaction as is your son. Even if they believe it's no longer appropriate for you to attend their wedding, your son shouldn't be ignoring your attempt to talk things through.
I hope that a resolution can be found, that you can talk things through with both of them and that you'll attend the wedding. Whatever happens I hope that you don't lose contact with your son and grandchildren because that really is a heavy burden to carry.