The boy is showing an early sense of humour it seems... or perhaps he just knows his father is really jokey and sometimes you just have to laugh at him.
I usually feed the baby while I'm sitting on the lounge with the baby lying on a pillow. At one feed time Tops was sitting in my spot and grabbed the pillow, so I handed him the baby. MrJ looked up, saw the wrong chest and just cracked up.
Perhaps he understood the joke or perhaps laughing at dad is a survival instinct, I don't know, but it was very cute.