I believe the term three piece suit doesn't include the shirt
I think it means jacket, vest and pants. (Unless your Welshie, in which case it means jacket, waistcoat and trousers.) The trousers should work with a nice polo shirt for a casual date. If there's nothing in Doc's closet, I'm sure he could buy a shirt someplace nearby.
Or maybe he could use a glue gun to slap an outfit together from a curtain.