Estranged

Unfortunately. I had a similar situation with my nephew. I decided to invite him and let him make the decision. But could you possibly contact him prior to the wedding and not wait for your event to be a reunion??? Best wishes

Not a similar situation but here is my opinion. Invite him. I agree it would be hurtful had he rejected but the reason I say invite him is because maybe that could be the olive branch to smoothing things over between your family or if not, it is better to know you tried rather than not invite him and then him talk smack about you not. This way you know you tried and your mom seems like she would be okay with it.

I think you should invite him. If he doesn’t come respect his decision but hopefully he will.

I think I would invite him. Perhaps using it as a leeway to mend certain relationships

I may be the odd one out here, but I would not invite him. My mother is my best friend and any one who is rude to her is rude to me, case closed. She's always defended me like the world was ending and I feel I owe her the same. She would never ask me not to invite someone, but I would do it out of respect for her. However, my parents are also paying for most (like 90%) of the wedding, so it would be rude of me to do that when they're paying, too. I know your situation is probably different, but that's my 2 cents.

It sounds like your mom is prepared to handle it if you do invite him. I would think it over and have one more honest talk with her about what you want to do: "Mom, I've decided to invite/not invite grandpa to the wedding, so I'm just checking you're OK with that." People are right, he may decline. If he does show up, you can seat them apart from each other.

If you can't imagine your wedding without him there, then you should invite him. At least give him the opportunity to attend, then the ball is in his court.

My FH and I had a similar experience with his aunt. Things got really bad and we still don't talk to her. We chose not to invite her to the wedding because we didn't want that stress on our wedding day. I say the best thing to do is weigh your options, maybe a pros and cons list. Ultimately that's what we did and decided that potentially having a family member there that would cause tension and potentially drama wasn't worth spoiling our big day

Before that time, pick up pen and paper, and write a letter, maybe once a month. Ask how he is, make it clear, " I know you and Mom have not been getting along, but that is a separate thing. You are an adult, and don't get involved in other people's issues. You miss him. A little about your wedding. Talk like you assume he is coming. If he asks you not to sit him close to mom, honor it. But refuse to listen to him or mom if they start in on you not inviting the other. This is my party, and I would like to see you there. Who else is invited, is up to me and my FI. It is my experience that with serious rifts in a family, and many friendships, trying to push people to talk, or get together, backfires. Acting like there is a truce for the time of your visit or party, as you would if you invited a couple who had a fight on the way to your wedding is better. You know nothing . You take no side. After a certain number of gatherings where they stay apart, they begin to lower their guard. Talking to others closeby, and listening, they are tired of the fight. Hovering. And long about this time, someone starts to talk to the other.
Your goal is to stay friends with both, beginning to end and not get drawn in. Start with a letter that lets grandfather know you care, and are not taking sides. Whether or not he responds, keep doing it. Whatever happens invite both.