This shirt is hard to see. It appears to be a sedated grey plaid with a pattern of darker charcoal stripes that run in sets of three.
In The Vessel (11x14), Dean wears this plaid in the final scene, when he and Sam are out on the pier trying to come to terms with what happened.
In Ladies Drink Free (12x16), Dean is wearing this shirt when Sam brings Claire back to the hotel room after she is bitten. He continues to wear it until she is cured.