Off the wall suggestion: would your son be the type to consider a year abroad as an exchange student? If you’re in an area with limited ECs, etc., a year abroad might help him stand out in applications. Plus, it can be a welcome change of pace for a mature student who isn’t challenged by their hs curriculum, and generally it is a life changing/shaping experience.
I can recommend the Congress Bundestag Youth Exchange program (CBYX), which is a 100% government funded scholarship program to Germany. Coming from an underrepresented state would increase his odds of being chosen, in fact (students are chosen by region with even distribution from the various states). I was a CBYX student in 2000-2001 as a high school junior and it was life-changing in every way possible. And it enabled me to escape my high school for a year haha. It shaped not only my life, but my college applications, where I was able to tell an interesting story and demonstrate my maturity/grit.
There is a similar program for countries that speak languages that the US State Department considers important for Americans to learn, called NSLI. Languages include Arabic, Russian, Korean, Chinese, and Hindi. Both programs are unique in that they offer Americans the opportunity to be exchange students in high school, fully funded. Most exchange student experiences cost 10-15K for a year (just program fees).
Just something to think about as an ‘outside the box’ idea 
If he is currently a freshman, I would look into it next year (sophomore), applying for junior year. It’s the easiest year to “skip,” provided you work it out with your high school that the credits from abroad would count/enable him to graduate on time.