Your son should be able to install MIT Scheme on the iMac, then follow the lessons in
Structure and Interpretation of Computer Programs (SICP).
This was the first-year computer science textbook at Massachusetts Institute of Technology for years before it was replaced by a Python text (which many old timers frown upon).
MIT Scheme/Lisp is not a popular modern computer language. However, the primary point of this textbook is to teach basic concepts, logic, analyzing and solving problems, etc. SICP was finally sunsetted by MIT in 2008.
Programming languages come and go. Basic concepts last for decades.
Your son does not need to tackle C#, C++, C, Python, Java, Objective-C, Xcode, whatever right now. If he does, there's a strong chance that he will not see the forest for the trees.
Also, keep him away as long as possible from IDEs like Xcode, Visual Studio and their ilk. He should learn how to program in emacs.