There's lots of good advice here, and I've upmodded the ones I agree with. But, really, it doesn't matter. Anything that lets him create something is good.
Some people here are concerned about "best programming practices" and learning certain lessons. He's 13. Just let him play, and if he likes programming, he'll continue learning.
I've heard several people say that when people start with PHP (or [insert language here]) they develop poor programming habits. Learning PHP or whatever else first could be the difference between becoming a casual programmer and a serious hacker.
Not yet and that is my point. That concept does not exist for anyone who has not programmed. I think we just have a disagreement philosophically in the proper approach. I think that this kid should be carefully molded into a coder who does things the right way because he already seems to have an interest. If he did not have an interest already then I could see why you would want to "ease" him into things.
I think your "let the kid be a kid" argument is valid though. It could be overwhelming and turn him off to the whole thing is coding appears to be too strict.
He's expressed an interest, but he still doesn't know what it is. If you hover over him and continually tell him "you're doing it wrong" he won't have fun.
In order for someone to decide it's worth their time to learn the best practices for anything, they first have to reach the point where they decide "I want to get good at this." The kid may never reach that point, and that's okay, too.
Kids express interest in lots of things. That doesn't mean they'll do any of them for the rest of their life.
Some people here are concerned about "best programming practices" and learning certain lessons. He's 13. Just let him play, and if he likes programming, he'll continue learning.