Roommate not paying... How do I get him out immediately!

If you are both on the lease, you don't. Your landlord does.

This depends on your relationship with him and his relationship with the landlord. Also local laws come into play.

Is he on the lease as a co-tenant? Or are you renting to him so that he is your tenant? Do you have a lease?

If he is your tenant. You can serve him with a pay or quit notice. This gives him 3-10 days (depending on local laws) to pay up or vacate. If he pays he gets to stay. If he doesn't pay and doesn't vacate, you file for an eviction order in your local court.