If your son continues to be interested in the Foreign Service, a good book for him to read is “Inside a U.S. Embassy,” published by the American Foreign Service Association. It explains all the different kinds of jobs people have.
https://www.afsa.org/inside-us-embassy
I would second the comments about the importance of learning a foreign language, especially a critical-needs language. Arabic would be great, for example. Check to see how many years his targeted colleges offer.
Veterans get preference for the Foreign Service.
My son is an FSO and has had assignments in Russia and several Middle Eastern countries. PM me and I’ll try to answer questions about the process.