I can’t answer the process question but if soccer were not involved why would MIT be his second choice ? Wouldn’t you look to find other strong schools, even if a tad lower academically where he could get a coaches support.
MIT is great but this has the potential to be the best four years of his life because of soccer. I would think if he has the ability to play soccer - wouldn’t a Harvey Mudd or JHU or Rochester or Brandeis or even a not quite elite LAC like Trinity or Connecticut College enrich his life more?
Walking on seems unnecessary for a kid that has the ability to earn a coaches support.
One can get a top level STEM education at many places.