Whatever his reason, the way that he has treated his partner and child is very cruel. If he wanted to back out, he should have done it before she moved, rather than putting her and the little boy in this situation.
The fact that he is vile to you, his mum, if you challenge him is another black mark against him.
It would probably be wise for you to keep out of it, as much as possible. Difficult, though. Obviously you love your grandson and worry about his wellbeing. There's also the fact that most of us still feel responsible for our children's behaviour, even when they are adults and must take responsibility for themselves.
apple trees not flowering this year. Anyone else?
The King is to resume some duties.