Heck, get the whole family involved!
Help plan the route that's the easiest from a terrain standpoint. Maybe Mom, Dad or you and your bro can drive the route together;
a "recon", if you will, before the big one. That way everyone will have a good idea of where he's riding, and know what to expect.
Make it an event, a spectacle, a big 'to do'! Video it and post it on YT!
And, just to be on the safe side; a document written by Mom and Dad giving your little bro permission to undertake his ride, so if
the local po po's get suspicious the letter and a link to above mentioned video showing the
entire family's participation will
hopefully cool their heels.
I dunno... personally, I think it's an awesome idea and if it's a dream of his, then stifling it is the worst thing to do.
There's worse things he could be engaging in or be influenced to do at 16.
Alan