My primary concern is that for now, it is a guitar. What happens in just over a year and a half when he is sixteen and can legally drive? Do they just buy him a car?
MIL has broken every personal relationship but a few over this kind of behavior and competition. She thinks that her "generosity" earns her immediate forgiveness for any and all offenses.
Exactly. Obviously, you know the situation with your MIL better than anyone here does... but this *really* struck a chord with me.
Essentially, you described my own mother.
You don't say, but is your DS the only grandchild, or perhaps the oldest? If he's not, does she "treat" the others like this? Because, in my own experience, it might be that she's viewing your DS as her "however many times it is, chance to make everything perfect". My DD is my mother's only granddaughter. Her two grandsons she treats as an inconvenience at the best of times (my nephew rarely, if ever, has anything to do with her as he's now working in Australia, my 8 year old DS also rarely has anything to do with her, because I strive to limit contact, but knows that he's "second best" to his sister). As far as my mother's concerned, my DD is her chance to "be a better mum" than she ever was to my brother and myself.
Also... does your MIL have any close friends? Maybe, if she does (mine doesn't; no one will talk to her, actually, which is pretty sad...), your DH could ask *them* to intervene, if that's not overstepping any etiquette guidelines? Sometimes we listen more to people our own age/from our own generation, than we do to those younger than we are, such as children and children-in-law...
I'm more than sure your DS will love the guitar that you and your DH have (?are) buying for him. Especially if he's old enough to work out what his grandmother's like and your past history of competitiveness with her. My DD worked it out almost a year ago, when she was 15. My DS worked it out when he was 3 years old. It's a hard lesson to learn, but sometimes...
They need to learn it.
I really hope this all works out for you.
(Incidentally, my DD becomes old enough to learn how to drive next year... and I'm terrified my parents are going to undermine me again and buy her a car, even though I've told her point blank that her wages from her part time job have to put to one side to pay for it... I pay her college fees and will pay her university tuition if she decides to go, but a car? Nope. Knowing what my parents are like, though, and knowing how many road accidents involve teenagers? It scares the *life* out of me!)