Be gentle in your approach. Almost as if you were meeting a fragile being for the first time. Use hand sanitizer before he comes around you and ask the parents " if you feel like you can talk with them " what is the best way to handle your nephew. If it all makes you uncomfortable, then I would suggest simply observing your nephew and how others are interacting with him. Studying people when I am unsure in a situation is the way I learn the best way to conduct myself. I have faith you will do well :)