I think you may be over-thinking the second instrument question. I mean that very kindly - I wish I had had a parent who was as interested in assisting me in “my path” back in the Dark Ages when I was thinking of music school!
It might help to understand how Music Ed degrees typically work. (Forgive me if you are already aware.). Generally (and I don’t know every program in every State in the Union!!!), students have a concentration in Instrumental Music, General Music or Choral Music. There will likely be at least one class in each area and more coursework in the area of concentration. Again, this is a very general statement as I don’t know certifications in all states, but with that caveat, the states that I am aware of have teacher license/certifications for K - 12 Music - technically a graduate can be hired for any position although most job searchers/applicants will stick to their area of specialization. You do occasionally find a general music concentration student teaching high school choir or an instrumentalist teaching general music. Or, in smaller districts, a one-size-fits-all teaching school band, choir and general music in some combination of grades. If the State in question requires a national exam like the Music Praxis or has a state specific exam like in Texas or California for certification, the candidate is tested on all concentrations.
To get a smattering of job postings for Music Teaching positions across the country, you might look the standard posting sites - Ziprecruiter, Indeed, etc. Not many districts post on these sites, but there are some. You can also check “Employment” on district websites in your area or the area your son aspires to just to see what types of music teaching jobs are out there.
Within an Instrumental Concentration, the student will be required to take a number of methods classes where they are taught how to play different instruments - brass, woodwind, strings and percussion. The student typically learns to play at a very basic level but primarily is taught “how” the ideal should be so that they can instruct students. For example, learning different embouchures, fingerings, basic acoustics of brass overtones, snare drum rudiments, bow holds, etc. The expectation is that the student is proficient on one instrument coming into the Music Ed program, not that they have mastered multiple instruments.
If your S is interested in another instrument, piano is very valuable. Most, if not all, colleges require piano proficiency via their classes or a test or both. Having a headstart before college takes a load off! Additionally, if your S isn’t already a singer, joining the school chorus or church choir might be useful as background.
Another area he could prepare in high school is theory. AP Theory tests are often NOT accepted in lieu of the college course. There are many approaches to theory teaching and schools generally want a demonstration that you are on board with THEIR approach. However, the background with AP or other high school class or online self-instruction can make the course easier or possibly allow a student to start at a higher level. Many find theory challenging so a solid background can be quite useful.