Have you ever discussed his situation directly with his probation officer? There may be inpatient treatment options available to him.
His conviction on a probation violation won't really worsen his record - he already has the underlying conviction. And he won't get better in terms of his substance abuse until he wants to. You have every right to tell him that he's not welcome in your house unless and until he's clean or, for that matter, that he's a grown-up and needs to take charge of his own life and find his own place to live.

