A lot of people are recommending Scratch, but I am going to recommend Construct 2. It is an awesome engine for making HTML5 games. It is visual coding so your nephew will have no problem with it. https://www.scirra.com/

Of course, listen to what pvaldes said also. Designing a game first will be better.

Also, I would recommend Teamviewer for you guys to use so he can share his screen and you can interact with it.