Unless you have a bona fide, legally enforceable contract that specifically guarantees you x number of hours per week, yes, he can. In the absence of such a contract it is completely up to the employer how many hours to schedule you for.
BTW, you can also be fired for refusing to do something, even if it is not "in the policy" unless what you are refusing to do is illegal or unsafe by OSHA standards.
Welcome back, jacksgal, you were missed.