Sounds like it will be an awesome adventure, and some great father/son bonding time. I did a bike camping run with my son when he was 14. Not a cross country tour, but 100 miles from home to a Provincial park where we camped for a couple of days, and did the 100 miles home in one day.
He had no prior conditioning for it. when you're 14, you just get on the bike and go, no conditioning required.
Now, 14 1/2 years later, I'm pretty sure I could out ride him.
Oh, and Merry Christmas. I'll be looking forward to reading your progress reports next summer.