Hi. Welcome to the site
Give him some time to adjust to his new surroundings. To find out why he's not eating, you'll have to tell us about your setup. How big is the tank? Do you have a uva & uvb light, water heater and filter? There are several reasons that turtles stop eating so with this info, we'll help you figure out what it is.
Another thing, I don't know how old your nephew is, but I suggest not letting anyone 'play' with the turtle. Kids are not exactly careful when it comes to things like that. Also, turtles are more 'look but don't touch' pets. I won't let my stepchildren (15 & 9 years old) or my niece and nephew (2 & 12) touch them. It causes undo stress and certainly won't help if the turt is already sick or not eating.
The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler