Kids of that age often show that behavior and it perfectly normal. My son did that a lot and often people in my family did not understand that. Instead of leaving him alone, they kept jumping on him and asking him to come to them. All of that frustrated him. Children become easily over-stimulated and this results in frustration. What you did with your nephew- leaving him alone and not overwhelming him by picking him when he didn't want to was a great strategy. When you do not apparently take interest n a child, you increase his curiosity and prompt him to come to you. So when a person does not give attention to my son and does not ask him to shake hands with him, my son himself goes to that person cos he wants him to take interest in him. Your next babysitting day with your nephew will go just fine. Just don't overwhelm him, stay calm and introduce activities to him without asking him to do them. If you want him to play with you, start playing on your own with a toy. If you do want him to try a food, eat it yourself first. Show him things but don't ask him to do something or even prompt him and he'll come running to you. Love and good luck!