The thing is that people who share indecent images of children online go to great lengths to stop Google and others from crawling their sites. They don't want their sites coming up on Google, since that makes them visible and that puts them at risk of a visit from the authorities.
He was interviewed on Women's Hour earlier today, and asked about whether his plans included a ban on page 3. It seems they don't and when pushed he seemed unable to give a cogent answer as to why not.