When I saw him wearing the cardigan I was reminded of a comment he made about his character in The Town (or a Mother's Son) about how his character was so conventional, wearing his "tragic" cardigan. (I think I'm not making this up -- that he actually said this.) Of course, maybe a cardigan is less tragic if it's worn over a tee-shirt as opposed to a button down.
My memory of his comment is that he said the Mayor was a sad man, and even his cardigan was sad. (Not as extreme as "tragic".) And Len's cardigan was sort of old and baggy, wasn't it?