You had coaches support. All students put forward for athletic Pre read have that support. Your sons acceptance was based on his academic strength and record along with the coaches support.
Stated differently if another candidate had the same application “score” as your kid the coaches support is why your kid was accepted.
I am advising you as a former recruited div 3 athlete and parent who has been very actively involved with advising numerous athletes. You will be playing a dangerous game.
Several recruiting cycles previously one of my sons friends had gone through the same process you describe your son completing but at MIT. He had some second thoughts and contemplated trying to rekindle discussions with Div 1 Brown. He reached out to a coach to “update” him. Unfortunately the Brown coaches brother in law was an assistant coach at MIT. The Brown coach had absolutely no interest given the athletes moral commitment and the player was never fully able to repair his relationship with his MITcoach although he did graduate from MIT.
You made a deal, had the benefit of a Pre read and support. It is morally wrong to leave that coach with an unfilled slot.
If you are suggesting leave his application at MIT out there and only act on it (if accepted) in the unlikely event the Caltech coach leaves seems a bit far fetched and would hardly be the norm. I would not however under any circumstances be calling or contacting other coaches.