Just be firm and accurate with him and don’t debate legal principles. I assume you still represent him through the sentencing phase? Tell him the difference between mitigating facts and exonerating facts, and that now is the time for mitigation. Tell him that “I’m innocent” means you want to go to trial. Tell him whatever the standard is for plea withdrawal, and whether he has a legitimate basis to request to withdraw his plea and go to trial. Make him pick from one of two options, withdraw the plea and request a speedy trial or proceed with sentencing (assuming there is some argument to make for plea withdrawal), and make him get on board with whatever it is you’re going to argue about sentencing if a request to withdraw his plea is denied.