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I'd say talk to different churches in your area. People always appreciate kids playing music. Also finding a Youth Orchestra in a few years could be a great thing for the student.

Your student seems to be coming from a musically active family. That´s a great thing and I´m sure it´s a pleasure to experience as a teacher.

I don´t know how community bands recruit new members. With the full-scale band programs in schools I should think few of them have some kind of "Pre-band" newbies-ensemble.
Over here we do, since there really are no practical music classes in public school here, so banding is everybody´s own business. Maybe trombone choir at church would be a way to go. I experienced everybody willing (not necessarily "able") to hold a horn will be accepted, even more so if the "willing-only" persons are working to improve.

I´m not sure what exactly you mean by "home-schooled" in this context.
Is the kid being taught at home instead of public school, or does he just not join band ?

Maybe you could get 4-5 other home school children together and form a chamber group? It really only takes two!

Many communities have a "home school band" or "home school orchestra". I sat in on a rehearsal for the Chattanooga Home School Band and did a paper on it.
Ditto the suggestion for community bands and church ensembles, also youth orchestras. Depending on where you live, there may also be a "pre-college" or "prep-school" program at your local college music department or conservatory.