Ehrlichkeit,
my advice (matching my other biases here) would be to engage your son in some worldly productive activity part-time. This should benefit his learning and, hopefully, direct him away from bad influence.
As it is, he has too much time to consider various directions, and the fancier the better. If he were to feel some responsibility for, say, paying his rent, then he will, hopefully, better concentrate his mind in his learning. Many chachamim of Rabbi Yehuda generation did that and were successful.