Congrats Clark! it is all good news, nothing to worry about :-)
I think that you should enjoy the moment and keep an open mind and wait for all information to come in (your son will probably get some more admission letters).
One thing to consider is finical aid, Harvard is by far the most generous one to middle/upper middle class families. MIT and CIT are a bit less. Depending your finical situation, that may or may not be an issue.
In April, admitted students will be invited to various campus for a couple days. Colleges are putting best shows to attract those who they already admitted (to keep yield high). MIT will give you a spectacular show (about how fun and how exciting the campus can be) and kids all love it. But keep in mind that will not be the real daily experience. Harvard puts less effort on pre-freshman weekend although most admitted student still come anyways. I am not sure about CalTech's pre-fresh weekend. I encourage that you and your son take the time and money to attend these events because they do give you a feeling of what is like.
My younger daughter DD17 was choosing between Princeton, Stanford and MIT last year. She kept her card closely until last moment. In the end, she choose Princeton mostly based on Pre-fresh experiences. She never regret it.
What major he plan to take? I assume that it is something math/science or engineering. Kids do change their mind frequently. My older DD18 changed major from English/Lit to Math and left me stunned. That gives comprehensive university like Harvard an edge because kids can change their mind without getting stuck.
