Since you want "reasonably priced", you're likely going to want something self-contained rather than van-supported. Self-supported trips guided by ACA run from about $50 a day for the three-month tours up to $110 a day for the one-week tours. Van supported tours are about 50% to 75% more expensive.
There are many other tour companies. Bicycle Adventures runs lots of tours, all van supported I think, and mostly fairly short. Even their "budget" tours are $335 a day and up. Some of their other tours run $500 a day. These prices make the ACA tours sound like a real bargain.
America by Bicycle is another large touring company. Their cross-country ride costs $225 a day, about three times what a van-supported ACA tour costs. Their shorter tours run about $330 a day.
All in all, the ACA tours sound like a good choice, huh?
If you're looking for a teen-specific tour, check out Outward Bound or Apogee Adventures. These are going to cost more. On an ACA tour, you'll son won't have many people his age, but there may be one or two others. The average age for ACA participants is 45.
How long of a tour are you interested in? Are you looking for a tour that caters specifically to teens? And exactly how old is your son?
Note that there are kids out there touring alone or in pairs as young as 18.