If he has done wrong things, take strict actions against him.
Deporting him an revoking his green card can be done only if he has done grave crime i.e something which threatens national security or murder etc, as far as my limited understanding of US laws suggest.(I am not legal expert)
He takes drugs (according to OP) which is not good, but its open secret that many pop singers take drugs.
He should be sent to rehab, if he really takes drugs.