This is beautiful, but heartwrenchingly sad. And Harry's expression, especially his eyes are so full of pain.
I love the way Harry is clutching his stomach, but I guess it's not the clue you were talking about? I mean, Harry is not pregnant, is he? After all his hands can't be "absently drawn in". My guess is the necklace and I'm not sure whether or not Harry's wearing a ring. It's probable. Maybe engagement ring? Well, in the portrait, at the left, there is also something lying on the table... maybe Harry's glasses? But I can't be sure.