Even if Maxwell tells the FBI that Andy definitely was involved with underage girls, he'll never do a day in court let alone a prison cell. His royal status gives him the very highest level of diplomatic immunity, he can't be extradited unless the government forces it. Johnson literally said during the campaign that "the monarchy is beyond reproach", Tories view the Royals as if they're Gods.
As horrible as it is his legal advisors have the right strategy. He's on the record denied the accusations (even if the interview was a train-wreck PR wise) and now he's completely retreated from public life. He'll spend the rest of his days in the shadows, despised yet comfortable.
He will probably never leave this country again. Be fearful of getting extradited if he goes somewhere that’s on favourable terms or owes America something.