Will he have taken precalculus/math analysis and calculus before graduating?
I sympathize. We looked into graduation for one of our kids. The solution ended up being a different attitude towards school itself with more emphasis on activities outside of school which, for that kid, were focused in one area of the arts. Boredom at school decreased with the decreased emphasis on it (though grades were maintained of course).
Does your son have a 504 plan or IEP for the ADHD and health issues? If not, make sure to establish one, because having one in high school will help with accommodations on SAT or ACT (apply early, it can take 6 weeks) and with accommodations at college. He should register with the disabilities office wherever he ends up attending.
Simon’s Rock was founded for kids like this, who are a bit betwixt and between high school and college.
Another avenue would be to address the ADHD and the boredom. Something like martial arts, dance, Tai Chi, yoga, kickboxing, Tae Kwan Do and so on can really help. Tai Chi is based on martial arts and really helps with focus. Is he on meds?
There are many ways to be around mixed ages and some of the above can accomplish that as well.
How is his health? Health issues can make college a little harder, but everything is doable even for serious chronic issues. However, if extra time at home will help resolve them or help establish a management plan, that extra time in high school is well worth it.
I know kids who did dual enrollment entirely at community college (or maybe they left high school early and went directly to cc). I talked with a really engaging young man who was bored in high school and was doing community college instead. He was taking flying lessons and getting his license at cc and seemed happy.
There are also online options to finish: check out Virtual High School. Some kids who were unhappy in our high school loved the online courses and the discussions with kids all over the world.
Finally, given his ADHD diagnosis, you could see his desire to graduate early as something to be discussed and restrained. We cannot tell if that is the right way to go: sometimes unhappiness with school is very valid. I do think that finishing the 4 years at his school is the easiest, least path of resistance, way to go but clearly he would need something else in his life to be happier with that route.
Colleges DO love outliers. He could also finish early and do something interesting for a year and then apply. Check out National Outdoor Leadership School for instance