When you son took the ACT and or the SAT, that is where most colleges and the military get the names and addresses.
Taking these tests shows that your child is interested in higher education, so the colleges, can then consider them.
They mean nothing. They are just putting their schools out there.
If your son is not interested he can just ignore them. These letters also do not guarantee that your son will be accepted if he applies. They are just trying to let your child know that they are probably someone that fits the at least minimum requirements.
IF your son is interested or at least considering some place like Harvard, West Point, etc..I would encourage him to send in for more info. It is an excellent first step in getting their attention. Is he a Sophomore or Junior in High School right now? Try to visit these campus this summer. go in Person and go on a guided tour, again, this lets the school know you are really interested.
If he is a Junior, I suggest he go ahead and ask at least 2 Academic Teachers, a Principal and or a Coach to write him a college recommendation letter, over the summer. The reason so early? It gives these people time to work on them and your son will not be so stressed in the fall waiting and chasing down these people to write them. These teachers may have him fill out a form telling about himself. It helps these people write a more personal letter. Certain teachers on Campus will be highly sought after, so they limit how many they write. Early applications are due by the end of the first semester, but Senior year is a very busy time for you and for him..
Let me know if you have any questions.