Welcome to ChristiansUnite! It sounds as though you have a typical toddler on your hands, full of energy and curiosity. I think it's a good idea to purchase educational videos for your nephew, also animated Bible stories. Be very cautious of the content of secular cartoons. You would be surprised at the subtle messages some cartoons contain. When I was in college, I watched cartoons one Saturday to critique the advertisements shown during children's programming. I was shocked to see Smurfs dancing around and chanting on a pentagram. I then noticed a door in the background that had an inverted cross hanging from the top!
I realize just observing the activities of an energetic child can be exhausting, so try to get plenty of rest prior to spending time with your nephew. It is also extremely important to spend quality time interacting with him. Play with him, read to him, take him outdoors and let him run around. If you are close to a playground or park, take him for interaction with other children.
Most importantly, make sure he feels loved.
Pray and ask the Lord to guide and strengthen you. My prayers are with you.