Really none of my business, but I'll throw my 2-cents out here anyway.......I would really caution him against riding solo across the US as a 17 year old. Not because of worrying about him getting into trouble, but because of his age restrictions. IF he DID have a problem, as a 17 year old would he be able to legally handle it? He would possiblh not be able to get medical treatment without an adults consent, wouldn't be able to rent a hotel room as a minor, or rent a car or truck if he had to because of a breakdown. IF he had any legal problems, he would still be considered a minor as well...very limited legal rights as a 17 year old.
The year of my 50th birthday was also the summer my son turned 18.....we did a 2400 mile/10 day road trip that May when my son got out of HS. Still one of the best times of my life. Even though we haven't done any other really long trips like that since, we still have done several weekend trips together, and I still enjoy them quite a bit. Try to get him to go along with you instead of doing the solo thing.....hopefully it will be a lifetime memory for BOTH of you.