best course of legal action

[colonel81]

we have a roommate who owes us just over four figures in back rent and as soon as we get it he is out. he has said many times yeah ill have it for you, but so far nothing. there are many other things we are going to accuse him of but dont want to until we get the money. what is my best course of action legally. how binding is a verbal contract, as he is not on the initial lease. is the best option getting a signature then sueing him when he doesnt pay this month. this is the last time im so trusting