Is it legal for my boss to decrease the PTO written in my contract?

1) If it is in an actual written contract for a definite period of time (e.g. a one-year contract) which is unexpired (still in effect), your boss *cannot* change your acrual during the term of the contract (i.e. until it expires): he is contractually obligated to a certain amount of PTO for the agreed-upon period of time. If he does change it, he will be in breach of contract; that breach would allow you to treat the contract as terminated (i.e. resign without penalty) or sue for breach of contract to get the additional days.
2) If the "contract" however is not for a definite period of time (it is open-ended) then it may be changed at will *going forward*: a contract which does not lock in benefits or compensation for a set period does not limit the employer's right to change the terms at any point, from the moment the change is announced to you forward. So in this case, the employer could reduce how much you accrue and you have no recourse. He still can't take away days you have already accrued or earned, since you did the work for them; the reduction is only effective, as stated, from and after when you are told of it.
However, in this situation, if there is no contract for a definite period, just as the employer could change your benefits or pay at will--or terminate you at will--*you* may quit or resign at will. A contract not for a definite time does not obligate you.