He's going to reach his skill cap soon (at around 17K, and he's above 10K for the moment), so if you plan on training for another year or two he won't pop that often, thus his value will drop over time. I recommend that you sell him after he reaches his skill cap; you'd have a higher chance of getting a better deal (as he's young, some people that don't pay attention to his potential, may consider him for training as well).