Yep I'm on my mother's accounts for the same reason. She travels a lot and I pay bills etc, transfer funds. You do have to go through anti money laundering. Proof of identity and address.
The only one that is a bit tricky is the credit union due to insurance included as you have to nominate one main person. So being the executor that probably helps.
You can give your son signing authority on your accounts, without including his name on the accounts.
He will have full access to the funds but you will still be deemed to be the owner.
Stipulate your wishes for the funds after death when you are filling out the necessary documentation at the bank. I was a joint account holder with my father and once he died the funds were automatically mine. If you don't want that to happen then tell the bank staff so you can have the correct forms signed.