Child has set separate screen time passcode

Hi. Please help. Since updating to IOS13 my son has managed to set a different Screen Time passcode on his device. This means that I can’t put the restrictions on his phone. He is adamant that he didn’t do anything with it and now I am unable to change it back. I’ve tried to switch it off from my end and then reinstate it but it doesn’t sync to his devices. All was working as it should prior to the update. The only thing I can think of is to restore his device back to factory settings but looking for any less drastic solutions. Thanks in advance.