Why does he want to be Course 15? Does he actually find the classes interesting or engineering too difficult? I knew three Course 15 majors. They all declared it because they couldn’t do well in anything else. They became very successful eventually, winding up at Harvard and Stanford for their MBA.
I think Course 15 is fine as a double major. MIT is hands down the best engineering school in the world. It would be a shame not to take advantage of that opportunity. Business concepts just aren’t that hard and can be learned quickly.
As a general rule, business majors aren’t highly respected. Even MBAs aren’t unless the degree is from a top 10 school. The best path is to study a discipline to gain industry expertise, work for 2 years, then get an MBA.