Does your teenager like cycling? If he's not a cyclist or used to traveling or roughing it, he might find it mentally grueling to wake up every morning knowing that he has to get back on the bike for another long ride. My advice? Stay flexible and open-minded, and let your son dictate the pace and agenda. Your job is simply to enjoy his company.
My grandson (then 14 y/o) and I went on a week-long bike tour of Prince Edward Island, Canada, 2½ years ago. We had a splendid time, despite getting soaked one night in a torrential rain/thunder storm that blew tents away and despite some mediocre support from the tour company. We averaged about 40 miles a day, which was fine--a former MTB racer, my grandson is a much stronger rider than I am. Moreover, 40 miles a day isn't bad at all when you have the entire day in which to cover it. Of course, my "lad" had a great time, in part, because one of the tour company's support people was an attractive young woman in her early 20's, and my young sidekick was strutting about like a rooster the entire week! I think he had a good time; as for me, it was the second-best vacation I've ever taken.