Thank you for your question.
Is this roommate on the lease? If so, the landlord would have to file an eviction notice against him. Your son can't lawfully kick him out. If he's not on the lease agreement, it's a different story, but getting him out tonight probably isn't going to happen if he won't leave willingly (and it never hurts to try that first, by explaining that he can get into trouble and violate his probation by having him stay there). If that doesn't work, then he should try this:
First, he should give his roommate a written notice to leave by a certain date, and keep a copy for himself. If the roommate doesn’t leave by that date (say, 5 days, or a week), contact law enforcement. If the roommate is not on the rental agreement and is not paying him any rent, a law enforcement officer may be willing to stand by to keep the peace while he persuades him to leave. You can also talk to the landlord about helping him make him move out. Since the landlord owns the property, the landlord could ask the roommate to leave and if refuses, the landlord could file trespassing charges against him. However, the landlord may take issue with your son having him stay there without permission if he wasn't aware of it!
If the roommate has been paying rent to live there, then your son will likely have to follow the requirements of Montana landlord-tenant law to evict him. That would mean he would first give his roommate a written notice to vacate, then if he doesn’t leave, he would have to file an eviction lawsuit against Terry. For more information about how to do this, see montanalawhelp.org/issues/housing/landlord-information.
The issue with the probation officer is more tricky. He's in a catch-22, and you know better how accommodating or not they are. If his probation prohibits him having a roommate, he can be violated when she finds out. But if she's pretty easy going, and you explain that you just found out about the roommate, and he is (voluntarily, hopefully) leaving tonight, she might let it slide because he came forward, especially if he hasn't been a problem for her and he's been on probation awhile. I can't advise you what to do, legally, as I'm not your son's lawyer. But if she's a stickler for the rules, I'd say he's got a problem then, because if he comes forward, she may still violate him, especially if she finds out about the drugs (even though he has a medical marijuana card). So he may be better off keeping quiet and convincing the roommate to leave. Problem there is, if word gets back to her somehow, even if he leaves, she may be even more mad that he didn't say anything. Like I said, I don't know his probation officer, and I can't give you legal advice.