Whatever's going on between the parents, I think the best you can do at this point is, if feasible, contact the mother yourself, preferably with your son's blessing, and very sweetly offer her a nice week off while her child comes to stay with grandma.
Or, if that's not in the cards, make yourself back off some mentally and re-focus on the other things that are (or could be) in your life instead. I don't mean to be flippant. It's just that when you're not one of the parents, there's really not much else you can do.
Knitting - raglan sleeve too wide. Help?
Electric buses arriving on our main bus route giving me a problem


