how do I remove a absent co leases from the lease?

[Paul Matteson]

Hi my name is paul. I am on a apartment lease with a former friend and he has disappeared with out payment and will not respond to me or the landlord. I know he is living with another girl and is just being difficult. But I need to pay rent and want to remove him from the lease so I can add another person too it. How do I do this and can I ask for his portion of the rent since he has been gone for upward of 3 months?