IOS device kid changed screen time passcode and claims he did not , help me catch him
First thing. Offer your son a non-threatening chance to help fix the passcode. Why fight someone when you can convince them you will win and gain their acceptance and cooperation after they break the rules?

That being said, just wipe the device if you’re out of patience. Connect to iTunes on windows, Mac. Erase the device and start over. 


https://support.apple.com/en-us/HT204306


As long as you are clear where the boundaries lie and if the device is for your conveneNiece or their convenience you can plan to lock it down again or get more skill at countering your son that may have more time to learn how to foil your boundaries.