I think that if you tell him to move out and he does without complaint you probably would not have to worry about him coming back with an illegal eviction case.
You should document the pot smoking or get a witness. I would go to the police station and try to find a drug detective or police officer that would work with you. See if they have a K9 unit that could sniff out the drugs. Then when you smell it again call them to come over right away. If they smell it also they would have probable cause to search his room. Even if there is none in his room it would scare the bejeezuz out of him.
And by the way, you are making a big mistake by not having a written lease that details all aspects of your tenants and your rights. There are certain rights you may lose if they are not spelled out in your contract. You should do an internet search to find the statutes that apply to rentals in your state and study them thoroughly and you'll see what I mean.
I personally always make my tenants sign a "Zero Tolerance Drug Policy" addendum, I bet you wish you had one about now.

