This is not "new evidence", it's a fundamental alteration to a witness statement, which actually undermines the prospects of a conviction rather than securing one. Can't see how or why the police would pressurise him into claiming there was a knife, sounds more like an embellishment of his own in the first place to get the police more interested in the case, which he's now getting cold feet about. The police are right - if he now changes his story he's admitting perjury, which is a criminal offence. By the same token if he sticks to his story and the judge finds he's lying, he could still be accused of perjury.