But the damage is already done years ago. He doesn't make any effort to speak to or have contact with his grandmother at all. So having no contact with her going forward because she's upset isn't some great hardship to him. He's been snubbing her attentions all his adult life. He's not committed some heinous crime with the evening invite, he's acting the same way towards her that he's always acted.
If someone goes their entire adult life not responding when you reach out to them with cards or messages, I find it weird and a bit deranged to keep contacting the person. I'm wondering why grandmother hasn't taken the hint long ago and stopped contacting him.
Given that they have no relationship, an evening invite seems appropriate to me, a token gesture because she's family. If she was any other random person he wasn't related to she wouldn't be invited at all, given there's no emotional ties. His own mother is seemingly fine with and understanding of the decision too and hasn't even bothered to respond to OP questioning it.
It's OP and his grandmother who have decided she has an important place in his life. The man himself clearly doesn't agree, he reached adulthood and cut ties. I find it strange that in those circumstances anyone would be expecting grandmother to receive an invite to the wedding. It seems like sticking their heads in the sand to me.
Constantly contacting someone who never responds, maybe downplaying it and infantilising him like oh he's a man they're bad at keeping in touch etc, instead of facing reality that he's choosing not to be in touch for whatever reason. Now they've had it spelled out to them that he's not interested and they're shocked, which is their own fault IMO for not paying attention to the very obvious signs he's been displaying for ages that this is the case.
Society as a whole needs to stop babying men and acting like they somehow can't stay in touch with people if they want to, like they're genetically forgetful or thoughtless or far too busy in Big Important Jobs, like it's a side effect of that Y chromosome to be unable to contact people. I think men are very good at centering themselves in their own lives and doing exactly what they want, so if a male relative isn't keeping in touch it's because he doesn't want to.