Roommate stopped paying. We're both on the lease. How do I get rid of him?

Tough one. Generally, a co-tenant can't formally evict another co-tenant under landlord-tenant law because you're not their landlord — only the actual landlord can file UD. Your options: (1) Ask him to leave voluntarily / sign a written agreement to leave. (2) If he agrees, ask the landlord to remove him from the lease. (3) If he won't leave and won't pay, the landlord may serve a 3-day notice for the unpaid rent — that hits both of you, and you have to pay the full amount to cure. (4) Some jurisdictions have a 'partition' or specific roommate-removal procedure for co-tenancies. Worth a 30-min consult before a confrontation; co-tenancy disputes are messy and you don't want to give him a self-help-eviction claim against you by changing locks or removing his property.