" He wrote me a check for his 1/2 of the mortgage yesterday and wrote "Rent" on it."
If you haven't cashed this check yet, wait to talk to a L. I'd hate to see you legally put into a landlord or lessor role which would make it very very difficult to get him out of the house.
If you cash the check, you might be accepting his terms according to which the payment is rent and he is a tenant.
