OP, I do not read this as “making this about you” and I don’t think that kind of criticism, based on very limited information, is helpful. I see you trying to understand the reasons for your kid’s change of heart and make sure that it’s based on positive attributes of the school he likes, not just cold feet or impostor syndrome. You’ve made no mention of “not letting him” choose the school he prefers; you just want to support the process and make sure his reasoning is solid and won’t lead to regrets. That’s more than fair and open-minded.
It sounds to me as if Mizzou is out of the running - I’m not hearing that he’d choose it over SU at the same price, and I’m not hearing anyone else (at least so far) propose reasons why he shouldn’t pass up Mizzou for one of the other two.
Hopefully the SU revisit (and accompanying discussion) will help to provide clarity. Can you buy time by paying Purdue’s housing deposit without committing?