I believe that the law in my state requires that the owner pays water and sewer. If he wants you to pay for it he can do so by just raising your rent. You shouldn't be paying a bill that is in his name. He is out of line. You can refuse to pay it but be prepared for him to raise your rent. I would refuse to pay it.

Generally this is an item that is in commercial leases where the tenant pays all utilities and taxes. In most states the owner pays water and sewer by law. (though he can pass than along by higher rents) Depending on the state there is a good chance this may be illegal.

Consult either an attorney or in most larger cites there are usually housing/tenant rights groups.