VS, I am sorry things did ot work out for you and your mother. Mother-daughter relationships can be very difficult- and 'blame' can come from either side, or both sides.
In this case, it seems to be that Gparents have gone way beyond what can be expected. Yes, we only see one side here- and the son may well have a different take on the reality. But sorry, if he refuses to get together and talk, then he has lost the argument already. And nver mind grand-parents- OK- but what about the children/GCs, who are the real and most important victims here.
Just because parents (GPs) are the parents, once children are adults- does not mean they are to blame, or are the only ones who have to apologise, make amends, walk on those eggs constantly, crawl and beg - or else.