Hi MZ,
If your son uses your card as a debit, then he won't need a note, nor ID, or anything else.
Most places have their POS systems set up to offer the customer a choice of whether or not they want to use their card as debit/credit. BUT, if your card is a straight out credit card(not a debit or bank card), that option will not be available. In that case, unless he has a card that is issued in his name from your account, *you* will have to be the one who uses it, and any retailer worth their salt(and their cashiers, for that matter) will check ID, and not let the user use a card that is not theirs. This is nothing personal--it's watching out for the customer to help protect them from theft and fraud. It also protects the retailer's liability from fraudulent/unauthorized charges.
You could send him shopping with your card, but he will probably come home with a story or two about how X retailer refused to let him use the card. I know some of them won't check, or won't care, but any of them worth a darn will. It's just good business practice, and looks out for *you* AND the retailer.
Why not go to an ATM and do a cash advance from your credit card to give to him? Easier and less hassle for him, and lets you regulate how much he gets from that card.
K. W