Gold Medal! Robbed! Apple!

That's quite a headline. Unfortunately simple games are easy to copy and a style is IP that is almost impossible to protect. I would tell your son that he should be flattered that someone liked his free game enough to reproduce it from scratch.

If he is trying to make a business out of this then this is a good chance to learn about how marketing is a good differentiator. If this is just for fun or for a portfolio then don't worry about it, and start working on the next project.