Aw but the good thing out of this is he's still coming ..
You have done the right thing: you invited him to be a part of and share a big event in Danny's life. Now its up to him to do the right thing and put his hard feelings aside. No matter what happens, your conscience is clean, and Danny needs to understand that his grandfather's behavior is not Danny's fault, or your and his father's fault. You did the right thing!
How old is he? Sometime people who is getting older, it become harder for them to write.
Well, From what I read, he is getting older, and maybe he doesn't like having no contact with his son and family. so he could be trying to connect with you guys again.
Maybe he misses his son. Granted, I don't know what happened to cause no contact, but My thinking is maybe he feels he is getting older and wants his family around him before he departs from this earth. That's the sense I'm getting.