Paste this in your browser.
http://reviews.ebay.com/PAYPAL-DISPUTES-RETURN-AN-EBAY-ITEM-OR-NOT-RECEIVED_W0QQugidZ10000000002187516
You can tell him you do not normally accept returns, but will take the shirt back out of kindness. He will need to pay the postage both ways and any ebay or paypal fees. He could also just sell the item himself.
Sometimes someone is bound to give poor feedback and there is nothing you can do about it, except return the negative feedback and put a comment on your negative feedback explaining the situation for other buyers to see. This could lead to mutually withdrawn feed back as well.