I don't know your situation, but the good thing about spineless uppers is that they are often as unwilling to work against you as they are for you. This PM is not actually this irrational. He behaves this way because hes become accustom to getting away with it. I don't know how much you "need" the job, but I'd try just telling him how its going to be and then sticking with it. Unless he has direct firing authority (that he actually has the political capital to use), he'll likely complain to upper management and get the same disinterested shrug you received. "He seems like a good employee, I'm sure you can work it out".

Caveat emptor of course. I have a sample size of 2 in my personal life and its worked for me once (but wonderfully so). The other time, it got me out of a bad situation that I should have been working harder to leave anyway (with good references even).