Worth looking out for a second hand Islabike (either a Luath 700/26 or Beinn 26 depending on size and preferred size). You'll pay more up front, but will recoup it when you come to sell. Most importantly, these bikes are very well designed and put together so your step-son should get a positive experience.
This is my second post recommending Islabikes this afternoon... I've no connection with the company; simply a very happy customer of at least six Islabikes to date (some new, some used, all good value).
Kevin