The problem with trying to use gap year activities to boost chances for admission is that your kid will be applying again in the early fall of the gap year (for ED) or late fall (for RD). That doesn’t allow for much time for any gap year projects, internships, etc. to have actually gotten going and impact admissions. The advantage of taking a gap year is that it would allow the entirety of senior year to be considered, including perhaps an increased GPA. But, the impact of the actual activities during the gap year would be minimal.
Michigan, Tufts, NYU, and USC are not likely or even match schools. Not because your son isn’t a great student, but because these schools are not likely schools for anyone. Tufts had an under 10% acceptance rate last year, USC was around 16%, Michigan was around 20% (for OOS), and NYU was around 12%. Early decision could increase the odds (at the schools that offer it), but even with ED, I would still consider them all reaches.