Matter of fact, I would find a picture of a DS and make my own chart out of the DS image with tick marks to denote his progress toward saving for a repair.
Is he to save up/cut out items for the entire amount? Or will there be some parental contribution of money versus in-kind contributions such as the money you didn’t spend on snacks at the grocery store? I ask because I would use different colors to denote where the money comes from. Red might be his birthday money contribution, Green might be a certain percentage of his allowance (I forget the word Dave Ramsey uses), Purple might be the money he saved by not buying snacks and blue might be the money mom and dad contributed because you want to see him progress toward his goals. He can tell by the percentage of color, where his money is coming from.
If he’s not paying for his snacks now, he won’t get the full concept of you not paying for the snacks unless you put the money in his hand so he can save it for the repair. I would probably take him to the grocery store, have cash on hand and when he doesn’t get an item he usually gets, put the cash for that item in his hand right in the grocery store. Then he can count up that money when he gets home, and color in his chart for that much money saved.