This totally depends on how well you train him. If you train him optimally (close to 10 pops a season with some secondary skills like IS), his value will increase. If you can't manage to train him as well as others would, his value will decrease because he'll fall behind on the optimal schedule.
Also, you have to factor in opportunity cost. If you want to buy another trainee, but have to wait because you still train this guy, that's lost value. If you have a spare training spot and you don't intend to buy a new trainee, you might as well keep training him (if you can do so optimally).
So, if you can't train optimally or you want to train another player, sell. Otherwise you can keep training him another season, but even then it won't increase his value too much.