What does your husband want to do?
It seems from your OP that he's so hurt by their plan to move 5 hours away from him that he doesn't want to talk to them again.
I can fully see why he's so upset.
5 hours drive away is a long way, that's a massive distance to choose to put between yourself and one of your children.
Particularly when you are doing it to follow another one of your children.
The fact that they didn't even talk to him about this is really shocking on their part.
It probably seems to him (pretty accurately, it seems) that they have chosen his sister and her children above him and his.
That's got to really hurt.
I think you should talk to him some more about what he wants now.
It seems that this is an ongoing problem and a sense of grievance that has been growing for years.
If they have already made up their mind to move away from what is (presumably) their home, where their friends and social networks are, as well as their son and his family, to follow their daughter then he might be right that there's no point in talking to them.
Their cowardice in raising this suggests that they don't want to talk about it and that they won't welcome it being raised.
Maybe it would make sense for him to let them know how he perceives their choice to move and the likely impact it will have on what remains of their relationship with him.
But I would forget letters and reasoned arguments and sitting down and discussing. I would think if he's going to have this out with them, he should just go and do it. Tell them how upset he is and how much they have hurt him.
They are taking a massive risk in moving in this way to follow their daughter. They are giving up everything they know, all of their support network and making themselves completely dependent on their daughter.