Using a sample size of one - esp. when that one school appears to be exceptional and not the norm - you could reach almost any conclusion you want, but it is well known that colleges are extremely cognizant of their yield (MIT certainly does care greatly about yield), act to protect it and often adjust accordingly. My original post had nothing specifically about MIT /Caltech, but simply about encouraging people to be considerate once they know a college is no longer in the running.
Generally, I would not feel comfortable drawing grand, overarching conclusions based on one high school. I am certain, for example, that no matter how many kids from TJ turn down MIT, quite a few students from TJ will continue to be accepted. But TJ and equivalent schools are the exception, not the rule.
Perhaps MIT and Caltech will continue to accept students from your son’s school regardless of the apparent trend since you seem to indicate that the school is more at the caliber of a TJ than the average HS. Perhaps MIT and Caltech don’t care that students at a specific high school repeatedly decline offers of acceptance, but many colleges do - that was the point of the initial post. And, except for the very few exceptional high schools, one never knows when a college may start reducing acceptances after years of students continually declining their offers.
Regardless, if a student knows he/she will not attend a college that student should decline an acceptance as a courtesy to both the college and other students - it is simply the right thing to do - and the crux of my original post.