Tenant law varies from state to state and city to city. Assuming your roommate is not on the lease, your best option is simply to ask him to leave. If he’s not on the lease, he may not have the legal right to stay in the apartment. Your jurisdiction may allow you to serve your unwanted roommate with a notice of termination, but that’s not the case nationwide.
Check the lease
You can’t serve the notice yourself – you must have third party not involved in the situation do so. If your names are both on the lease, read the lease terms carefully. You are both liable for paying the rent, and he has as much right to stay in the dwelling as you do. You may want to discuss the situation with your landlord.
May require legal help
If you can’t pay the rent on your own, your landlord may evict both of you. That requires going to court, but if your roommate comes up with the rent in the meantime, he has the legal right to stay. If simply asking your roommate to leave doesn’t work, you may require legal help.