This would create a constitutional crisis if he outright vetoed Parliament’s bill. Constitutional monarchs are not supposed to exercise the royal prerogative against the advice of their ministers. The best he can do without subverting the democratic process is to, like King Baudoin when the Belgians legalized abortion, to declare himself temporarily unfit to reign, thus preventing himself from the ability or duty to sign anything.
Of course, we could see a break of constitutional convention, but that would cause a constitutional crisis and possibly lead to his being stripped of even more powers (like what they did with the Grand Duke of Luxembourg when they legalized assisted suicide, because the Grand Duke, a devout Catholic, refused to sign the law).
We can see why JESUS said that to enter heaven one must be like a child. As we can see this child is smarter then many adults out there.