There are a lot websites that can help, I am currently 15 and i started programming at 13 as well.

I have a few recommendations, as a 13 year old i started learning python. It was my first programming language but if your son wants to learn a different programming language that's completely fine. If he is interested in a different programming language I would recommend him java or c.

I would recommend CS50 on edx.org for your son. This way your son can be introduced to various different fields of computer science. CS50 uses a lot C (programming language) but if he doesn't feel comfortable with C or feels confused, there are a lot other programming languages that CS50 introduces a lot of other programming languages as well, that he can try.

After doing the CS50 course if your son is not interested in learning website programming but has problems with C, i would recommend he learns python first.

Now these are the books I used to study programming and it has worked out pretty well for me : Python : http://inventwithpython.com/ (This website 3 books on it,a ll of them are free. You can pick any book you want they are all really good and all of them free.)

Java : Head First Java (In order to study this book, I would recommend he learns a programming language first and gets really good at it. Because I tried reading this book with no prior experience and it wasn't pleasant, I struggled a lot)

C : The C Programming Language 2nd Edition (I haven't read this book myself but there are great reviews on it)

I hope I have helped you out because I rarely answer question due to my lack of experience and please forgive me for my grammar and spelling mistakes.