Is he willing to go anywhere? Are you willing to pay full private tuition, which is now about 90K/yr?
If not, then you can start narrowing your list just by geography and cost.
Usually, by the time that someone is doing a performance music major, they are intensely focused on one instrument, or doubling on closely related instruments. For example, trumpet and flugelhorn. Sax and clarinet. Flute and piccolo. They wind up focused like this because to get good, they need to practice many hours a day, 7 days a week, nearly 365 days a year. It’s too hard to do 2 hours/day trumpet plus 2 hours/day guitar and everything they need to do for school, although some manage it somehow.
Start with a conversation with his private instructors for guitar and trumpet. If he doesn’t yet have private instructors for these, a year with them, starting immediately and intensively, may still greatly improve his auditions next year.
If he doesn’t have a very high level jazz ensemble at his school (which a small private school just will not have), it may not yet be too late to get into a local regional ensemble, if they have one. Playing in a high level jazz band is an invaluable experience in jazz players’ development.
Is there a local conservatory with a high school program? Is there a local conservatory where a jazz instructor would be willing to start a high school jazz program, now, for this year?
My kid was jazz and classical, but was applying for classical. Kid made a spreadsheet of all the schools that had a teacher for their instrument, with whom they would like to study. They eliminated some based upon teacher was not a good match - personality, playing style, spread too thin. They found out about all this through people they’d met at festivals/camps who were ahead of them in school, also from internet research, and listening to their recordings, because you wind up sounding like your teacher. My kid already had done very well in national competitions, and so knew that they had a chance anywhere. Then they eliminated schools based upon not having the academics available for them that they wanted, and locations they didn’t want to be in. Baylor, for example, was off the table.
Once they’d factored in teacher, location, academics, they wound up with fewer than 10 schools. They needed a great audition AND great academics for the places they wanted.
I think that your focus right now for your junior should be on jazz music preparation, rather than schools. Start thinking about schools next August. Right now, focus on private teachers, a great jazz band, if possible, and his improving his skills, whether it’s performance or composition.
When it’s time, know that some schools accept students with great auditions and somewhat lesser academic stats, while some won’t. For example, U Michigan accepts jazz students with great auditions and okay stats, who wouldn’t have gotten in if they hadn’t been going into jazz. And some schools which are not academic powerhouses are very famous for their jazz programs - U North Texas is an example of this. But for now, focus on his improving his jazz performance and composition.