My parents tried the whole punishment thing with me....totally useless. I didn't like being forced into things so I just took the punishment. They did manage to force me to get a job in grade 12 though by saying that if I didn't, they'd kick me out (and I believed them). Ultimately I appreciated it, but I definitely resented that for a while. They didn't try rewards but I don't think that would have worked either. Hmm...what did work?
-my parents told me from a young age that they couldn't afford to send me to school and I'd need good enough grades for scholarships if I wanted to go
-I wanted to be a vet for a while and I knew I needed good grades for that
-more than anything it was just that I felt bad when I got a bad mark so I put in enough effort to not do badly but not a bit more than the minimum needed....
I guess I can't say much other than in my experience ISTPs are hard to motivate externally, they have to want to do it first....so if there is no internal motivation, I'm not really sure what you can do. Just make sure he's aware of what, realistically, he needs to do in order to get where he wants to in life....but really, at 12, that's still a long way off from mattering. It's only in the last few years of high school that it'll matter, and then only marginally.
Edit to add: something to watch out for, if you try to scare him or exaggerate and tell him something like he'll be working at McDonalds for the rest of his life if he doesnt get straight As, he'll know you're full of it and then won't take anything you say seriously. Be realistic in your assessments and he'll be more likely to believe you...or at least that's how I am.