And the fun thing: it's literally just a property of the standard "action" in Roblox. So super simple even if he doesn't use Lua but just the UI. I.e. instant gratification. And he can change the number and Roblox will automatically show a progress indicator for the "action". (yes I've done some developing coz the kids wanted me to build a game for them. "daddy, daddy, I found this pink cow! You gotta put it in!" - I never finished coz it got boring but it was a lot of fun).

He can blow up (just super size) the models and maybe find some "motor" asset someone made and published to actually put under the hood or just next to the car, which he needs to repair too. A blown out tire? Go find rubber or a new tire? Lug it back to the car first. A jerry can asset someone made that he needs to find at a gas station (maybe someone already made that asset) to fill up the car. Like "if Jerry can is in inventory - inventory is already pre built in roblox just use it - then long pressing the action key will fill up the car". Find a "lead pipe" asset to use as a weapon. To begin with use one of the premade with animations and everything swords. The learn how to make your own.

Since it's all for learning nobody has to care about asset licensing or money. When he does, it's a teachable moment.