We are a family of three with a 9 year old. In our current camper, he is very happy sleeping on the jack knife sofa. We are currently looking at Grand Design fifth wheels - lots of choices in the 30 ft range. We like the rear living layout with opposing slides and build quality/reputation - the one we most likely will settle on is the Reflection 303RLS - although there are shorter models.
I wouldn't get a bunk house for one child, or based on the chance he may want to bring a friend. They could both sleep on the sofa bed. While camping, we don't make him go to bed at the regular time - we all go at the same time, and besides if he wants privacy, he can use the front bedroom until it's time to sleep. I'm betting that your not going to bring his friends on a 3 month long trip.
Good luck!