This is not a situation where the bank can intervene.

If the buyer opened a return case stating the item was 'not as described', then your son should have accepted the return, even if he didn't agree with it.

Ebay don't see the item and they do take the side of the buyer.

If your son did not accept the return and didn't provide a label promptly, then the buyer can ask Ebay to step in.

What inevitably happens then is that Ebay refund the buyer from the seller's funds and tells the buyer they don't need to return the item.

Your son should communicate with the buyer and ask them to return the item.

Your son would have to provide a label or postage for the buyer. As far as Ebay are concerned, the buyer does not have to return the item or reply to the seller.