While I agree with all of the above in principle (and even I have trouble reading my own code at times), this part was specifically in response to the section about ‘code optimized to irrecognizability’ and should not be taken as a general statement on finding other people’s code incomprehensible. Deliberately using non-descriptive naming is unfortunately a thing, although thankfully I rarely seem to encounter it anymore.
I’ve been working with software for 15 years and still feel like this when faced with a new codebase - it simply doesn’t want to make sense to me. As others have stated, codebases are living things, and are as much a map of previous developers minds as the are about being functional. The older a project is, the more convoluted and obscure the structure becomes due to changes, adaptations, new features and changing contributors.
Some developers seem to enjoy making their code obscenely difficult to understand, either because it actually makes sense to them that way, or because it makes them feel smarter. These projects are better left alone for the sake of your own sanity. If you encounter dozens of header files, walk away. C (or C++) are high performance languages, and projects are using that language for a reason. If you have no experience with them, the result is very unlikely to make any sense to you.
I’ve also found it quite difficult to find any project small enough to help on. The large projects have many contributors, and any manageable bugs are quickly fixed, leaving only the stuff that no one wants to touch.
Is there some sort of hobby you enjoy, where an open source tool is (or could be) used? The more obscure the better! Having some prior understanding of the subject usually makes understanding the codebase a little easier.
If he likes working with physical objects as well (robotics) I can’t recommend the Lego mindstorms or education series enough. The standard interface is very similar to Scratch so he’d feel right at home, but they can generally also be programmed with more traditional programming languages if using building blocks becomes too restrictive.