My son reset his password to his phone and forgot it. Now he’s locked out.

Assuming his phone is updated to iOS 17, he can follow the directions in the link I provided below to use his previous passcode to get into his iPhone. If his iPhone is not updated to iOS 17, his ONLY option is to erase his iPhone using the directions in the Related Article Link at the top of this page, which brought you here.

Temporarily use your old passcode when you forget your new passcode on iPhone or iPad - Apple Support

I am assuming this is the phones lock screen password that he forgot. If this is so, your gonna need to erase the phone and set it up again. Get a PC with iTunes or a Mac with iTunes and put the phone into DFU mode by pressing volume up once and volume down once, then proceed to hold the power button until the phone shuts off. keep holding it until you see the apple logo, keep holding it until you see a picture of a computer with a lightning cable. Then, connect the phone to the computer with iTunes. from there click restore iPhone and if it asks you to restore and update, click that. this will erase everything on the phone and your going to have to set it up as new again. If its the iCloud password he forgot, then your going to need to call apple support and and show them proof of purchase of the phone so they can iCloud unlock it for you. hope this helps.