I’m not sure that MIT accepts SAT scores taken in the 8th grade. Check with MIT admissions.
MIT admissions is a holistic process and as such you can consider SAT scores more like a gate. Get a decent score and it is on to the the next gate. 790 is certainly going to get you thru that gate, so I wouldn’t stress about trying to get a higher score.
My recommendation for MIT or any other college would be to challenge him in his studies in the classroom and outside the classroom. Have fun and do what interests him.
Read the MIT admissions blog on “applying sideways” : https://mitadmissions.org/blogs/entry/applying_sideways/