If you were in the UK, I'd recommend looking in to a Monzo card. It's an online bank account coupled with an app. You can top it up with android pay. Your son could use it in stores like any other debit card, or could use it to draw out cash at an ATM if needed. Obviously don't know your son's age, but the app is pretty great because it automatically classifies each transaction and you could use the analysis as a way of teaching saving and budgeting. Unfortunately, this card doesn't (yet) exist in the US. I've googled for similar cards for friends of mine in the US, and they have similar ones. One example is called simple . Not sure if it fits your need, but worth checking out.
If your card worked before it was only on accident. This happened for me with Chase. When I got a new phone and tried to re-add my card, it would not work. Chase finally announce support earlier this month and I was finally able to add it back. My bank for my checking account still does not support Android Pay.
Your bank has to allow the card to be added. At least that is my understanding. The link below is a list of "participating" banks.
Ok, now I have a new scheme (tried the create another user account and decided that was too much).
So now I have smart lock set up and operational. I learned that when you have the lock screen in front of you, you swipe up to wake up the phone. If you want to use Android Pay, don't swipe up. Long press the unlock button at the bottom center and that will give you a locked screen to unlock.
I've actually installed Screen Off as a back up way to get to a scree that requires my pin that Android Pay will like.