Most Helpful Guy
it's our nerves triggered by emotions. He could be passionately in love to the point that the excitement of his affection for you dilates his pupils. It's like being nervous for something important, you sweat, your face goes red, you look flushed, your pupils dilate.
It's all emotion driven, I'm no expert and I can't say what exactly he's feeling. But I wouldn't spend too much time reading into it and would rather enjoy each other.