Perhaps some black and white statements will set your mind at rest:
-
The MIT coach is not interested in your son! That means it’s highly unlikely there’s a walk on spot.
-
Even if the coach at Caltech leaves, your son enters with a spot on the team. Some teams do make cuts of upperclass players, but a) your son isn’t an upperclassman and b) even if he were to get cut, you wouldn’t know it until he starts college – and this is true at every school, of course, so hedging your bets by applying to other schools doesn’t help here.
It sounds like your son has found the sweet spot of a great academic school plus the ability to play soccer there. That’s fantastic, and something that shouldn’t be taken for granted!