[Clodfobble]

Honestly, I think in the end, he will be getting a Forces of Valor truck. It's just a question of when, how, and how much resentment he builds up about it beforehand. Vacillating between absolute-zero materialism with mom and dad with spurts of ridiculously high materialism with MIL has got to be hard on the kid. It's my humble opinion that at this point it would be better to get him the truck yourself before your MIL does, which will only reinforce his idea that it's because she thinks he's deserving of it and you don't.

But not for free--I'm with Ali on the 'plenty old enough for chores' idea. If he doesn't really understand money, you can put it in a currency he does understand. Even my kid understood a little picture chart with ten blank spaces and a picture of a Hot Wheels car at the end of the line. (Still decided he wasn't willing to do what was necessary to fill even one square, but he definitely understood what he was supposed to do.) It doesn't have to be chores that are actually useful to you, or things he has to do by himself. Maybe tell him he can wash the car with you for a star, or he can help Mrs. foot fold the laundry for a star, even if he's not really making the process any more efficient. Or he can read a certain number of books to the mm.

He may never be as completely non-materialistic as you want him to be, but he can certainly learn a sense of working hard for it in the process...

Edit to add: I just had a flashback to my childhood. My parents never did this, but my best friend across the street would regularly be sent to the neighbors on either side to ask if they had any chores that needed doing. She would do a kid's crappy job of raking their leaves or whatever, but work her ass off doing it, and the neighbors didn't care because they were going to have to do it anyway and it was funny watching the neighbor-kid fight with a rake that was taller than she was.