You're not being selfish.
He's been asking you to lie to his mother for months (or at least engage in creative omission/misdirection when it comes to telling her about his lack of work). So of course she charges you for rent; she thinks you can pay that amount. Which, I am sure you realize, is a rather small amount in much of the US (don't know where you are, but a $200/month deal would be pounced on nearly anywhere).
$200/month and lie to his mother, or $0 (I assume)/month and live with your parents and cut ties with the boyfriend.
Oh, and it's his idea for you to earn and save enough to get a car?
He just wants a new car and a rent-free life, all while lulling his mother into a false sense of security about his independence and maturity level.
I think you know the answer to this one already. Nowhere in there have I seen anything about him contributing to anyone's future, and the idea of him getting a job with housing for a few months (to work in a summer camp? A ranger station? Something else?) is only skimming the surface of being responsible.
He's a tall kid, and not much of a partner. Sorry.