Is PTO required to be doled out "as accrued" or can I ask it up front?

My employer, a doctor (small business with only 4 employees) gives us 40 hours of PTO (includes sick, vacation and personal) per year but requires that we accrue it at the rate of 6 hours per month. Then, if we need time off we haven't yet accrued PTO to cover, we have to take it unpaid. He calls it an "incentive to come to work". But with the winter weather and cold seasons coming, my coworkers and I will surely need more PTO than we have accrued. We have children, snow closures force time off, etc. Last year I lost $300 on ONE paycheck that he refused to cover PTO for. What are the laws on this? What right do I have? Thanks.