Seriously, if it's that bad, then you're going to have to be the bad guy, and the sooner the better so that he gets the message now. If your MIL gives him that much money, then your ds needs to know that some of that money will go into an account. I'd have a serious issue with my 5yo becoming so materialistic at the hands of my MIL (not that that would happen - but that's a whole other story

).
I really want to teach my children about money, and even though a 5yo won't understand immediately, he can start now before the "gimme" habit forms too strongly. I saw a guy once a long time ago say that he taxed all of his kids money as family tax so that kids would start to understand the value of a dollar. That money would go into a pool for family extras, like vacations, lessons etc. It may seem harsh, but in this case where a lot of money is involved, pretty reasonable. That's why I really like the idea from pp of using some of it for stuff he can do now, like camps and lessons and sports, which all add up. He also sees something for that money now rather than 13years from now, which he can't understan. The focus moves off of things and onto experiences.
This is your ds, and even though it's your MIL's money, she's teaching him how to be greedy and excessive and imho, it's way more important to stop that than worrying about her feelings. Let her pout, and then give her $50 of her own money and tell her to take ds to the amusement park

. Just kidding
