Is it okay to renegotiate rent on a month to month agreement?

You're being reasonable, and your roommate is being greedy. You already know this, I suspect, but you need reassurance for some reason. Work out a more fair arrangement on paper, confront him again, (reminding him of your original conversation), and show him your proposal. Ask him to read it, and respond. Be friendly, calm and reasonable. Tell him you would HATE to have to move somewhere else, because you so enjoy his company and the apartment, but that you really need to have a more equitable distribution of rent expenses in order to stay on.