This is insane. Knowing the situation, would you go to the wedding ?
I would not go. If this guy wants to sweep his past under the rug and not tell his fiancee, fine, but I would not participate in the deception.
A similar situation happened to me. My best friend and I crossed the line when we experimented with sex. Well, it opened Pandora's box. It was bigger than either of us could have imagined. I had no idea that I was gay before I realized that I had an attraction to my friend, but had no problem with it. We were really happy. We used to suck and fuck daily like 2 tigers in heat. For 6 years, we had a great time. Despite the fact that we lived together and were lovers for all those years, he was a closet case and didn't want any of our other friends to know. Sorry bro. As close as we were, no one could miss it. His biggest issue was not wanting to disappoint his parents.
So one time his dad set him up with a date the daughter of a golf friend. First date, she got pregnant. I found out a month later about the shotgun wedding from his mother on the phone. The coward didn't have the decency to tell me. Mom was gloating (bitch!). Friend asked if I would drive him to the wedding in a nearby state. I said sure. He mentioned that his fiancee was a born again Christian and he would no longer be able to drink and party. Sounds like fun (not!). After he left that day and I told a few of my friends who had know both of us all along, one guy told me that I should dress like I'm going to a funeral and wear a black sequin dress with a gaudy black hat with a black veil to the wedding. When the preacher asks: "Does anyone know any reason why this couple should not be united in holy matrimony?", that I should run down the aisle toward the altar and announce: "Yes! He's been my lover for the past 6 years!!". Despite being distraught over the situation, I laughed my ass off. It became a running joke with a few of our friends who were also dropped cold because of his "new religion".
As it turned out, I never heard from him for 2 years. I never got my chance to attend the wedding in drag. L.O.L.! Seriously, I was devastated. I buried myself in work and college. Got straight A grades. Years later, he shows up and announces that he is getting a divorce. He had 3 kids. We started to hang out again, and he became reacquainted with our old social circle. He apologized for how things went with the shotgun wedding. I took the high road and didn't make him feel bad about it.
In spite of our renewed friendship (no intimacy), I moved to San Diego with my best friend (female). A few months later, he called and wanted to come visit. We had a great weekend going out to eat, going to the beach, etc. The first night, I told him you can sleep on the sofa or I have a king size bed. He opted for the bed. One thing led to another, and we hooked up. The funny thing is that after the amazing chemistry that we had previously, I couldn't keep it up for him. I was in amazing physical condition from distance running and the gym, so there was no medical reason for the ED. It was emotional. I realized at that point that I no longer had that special feeling for him anymore. That ship had sailed! So, finally I gave up trying to fuck him, flipped him over and gave him a blow job that rocked his world. My remark was: "Not bad after 8 years, huh?"
He ended up moving east and we lost touch for about 10 years. Then one day I did a search on the internet and found his phone #. I called him. The tables had turned. My attitude was casual and strictly looking up an old friend. He proceeded to tell me how much he always loved me and what a great friend that I was. I was flattered, but it was too late. The past was the past, and that magic flame within me would never reignite.
We still keep in touch occasionally. He married for a second time, but I can tell he isn't happy. His wife loves me, yet I wonder how much he has told her. A year ago I drove over to visit them. At one point, I told him that I was devastated when he got married the first time. He acted surprised. I told him that I would have been his partner for the long haul if he would have been into it. He hinted that he would like to hook up, but I let it go over my head instead of flat our turning him down. Sorry bro. You had your chance and blew it. I'll be your friend, but that's as far as it goes.
Everything turned out the way it was supposed to. I fell in love 2 more times since then, although we never got past being friends. The important thing is that I realized that lightning can strike more than once.
I would send a card and maybe a gift, but would decline the wedding. Make plans to do something else and not be available. Great that he is tight with your family, but for your own sake, you need to put some distance between you and him. You can still be cordial, but it's time to think of yourself. My honest opinion is that his marriage won't last, and one day he will look you up again. By keeping a safe distance, you will never feel guilty for his marriage not working, and he'll never blame you for it. Best of luck to you!