Well, I am going to post from the "other side of the fence" so to speak.
I do NOT see a problem with him going to a family wedding with his ex at all.
I am in a similar situation except I AM the ex-wife, lol. We have a family wedding coming up this summer (our oldest son) and I want both my ex and me to attend TOGETHER in support of our son. I don't think it's appropriate for either of us to have a "significant other" present at the wedding. We should be there TOGETHER as our son's parents, and nobody else needs to be involved. I do NOT want to have my ex and his girlfriend sitting together in one pew and me as the mother sitting in another pew.
I feel very strongly about this so I do understand fully where the OP's boyfriend is coming from.
I also have a great relationship with my ex. There was no reason for us to argue or fight over anything, heck, we never fought while married, why would we start after? lol. We've been very amicable, and I have no problem with picking up the phone and calling him or texting/emailing him on a regular basis to touch base on our three sons. I'm thankful we've been able to remain friends for the sake of our children. Nobody should EVER interfere with that.