A parenting plan can be changed in any state, as far as I know. Most states want there to be a _reason_ for the change, otherwise people would just go back to court every six months hoping to change a decision that was already made. If his ex-wife is not allowing him to see the child, then that is a good reason to go back to court and then while you are there you can also ask to have the parenting plan modified. You should have a good plan already set up to present to the judge. Don't just go in there and say "we want more time" because, unfortunately, that's not enough.
Can you give us more details? What does his current court order say? How long has she been cancelling visits?