How do change my email back to from my son's email to mine on Google payment like Google store My son put his email into my Google purchase account and I cannot change it back to mine how do I do so
To change back to your payment method, open play store open the menu on the top left, press on the email address, and it should give you the option to switch. 
(That's if you added your account to your phone. If you didn't, follow the steps below and you'll see a way to add yours )

If you want to fully remove your son's account from your phone, go to phone settings>accounts>Google>[his email address]>three dots menu>remove account.