Congratulations to him on those grades.
I don’t know if business schools factor undergrad majors into their decisions, but here are some things to consider before deciding to be Course 15 instead of double majoring or doing a single major in engineering.
He’ll have more fun and more free time if he does 15 only. He won’t bond as well with the rest of campus because Sloan is physically separated from the other buildings and because there won’t be opportunities to suffer through problem sets and exams with engineering students. Don’t underestimate this. MIT alums are tight knit because they respect what they’ve gone through together. He’ll know a small fraction of what an MBA who majored in engineering knows. There isn’t much to a business curriculum so I can’t imagine what he’ll learn in business school that he didn’t already learn as a Course 15. He’ll risk less respect from his schoolmates. Some people wouldn’t care but obviously you do. It might be tougher to get the exact job he wants after college graduation. It might also be harder to tell a good story when applying to business school about why he wants an MBA.
I’m splitting hairs. As I’ve said, he’ll do well regardless of his major. A career in business is more fun, in my opinion, and more lucrative. I’ve been recommending MBAs to anyone asking for my opinion.