yup, it's a nice tutorial, which shows you, that when you keep certain rules in your head, ANYBOBDY can draw (besides attention :) ) decent pics.
it's just a matter of practice practice practice.. The biggest "mistake" i see young artists make, is that they grab one piece of paper, and then think and try: "i'm going to make my best drawing ever!" and then spend hours on one drawing. doesn't work that way! (ok, for some really mega talented people ,it may...) but if you want your son to be consistent in his drawings, make him do a figure, just rough sketch, grab new paper, do same figure again, this up to 10 times, just between every sketch, look at it, what can i do better next time, what should be there next time, what should i leave away next time? etc. this way of working has a few BIG advantages: it makes you LOOK at what you did (everybody see's but only a few know how to look, there's a big difference there!) also, it is a property of a good artist to copy a drawing on top of youre head. the better the copy (with improvements) the further an artist is.
i didn't come up with this way of working, learned it on school / from other artists / learned the hard way.. Also, Rembrandt (the famous dutch painter) is celebrating his 400th aniversary, and they made a very nice detailed docu. about him, and guess what: Rembrandt did exactly the same! only on another scale though, for one painting (he has loads of famous ones) he did thousands (!!!) of sketches. thats thousands of sketches for just a few figures.
talking about jalous-making talent: nice detail was, that rembrandt used exactly 7 lines just for one face, with those 7 lines he got EVERY emotion he wanted: someone very angry, scared, happy, emotion-less, sad, etc. with just 7 lines per face. thats damn impossible! really it is: i tried it right after the docu for 2 hours.. (try it your self..it's good for a laugh..also, look back on tutorial, all those rules, just 7 lines, uhuh! ) best thing was, that when you compared ALL his sketches (for that one painting) to the actual painting, you could easely reconize the faces in the sketches were exactly the same as on the painting. really amazing. makes you think about the quality of artwork created by artists now-adays...
again your son has talent, but as everybody else, he needs some guidelines. Make him LOOK at his drawings, evaluate, do it again, and again, and again... in the long run, he will develope what we call an artists eye. this means he won't be looking anymore, but he will SEE (again, BIG difference there!).
once he has "the eye" it will be much easier for him do draw real people, and well, any other object really... first from foto, later after years of practice, in real life as well (nothing more difficult as drawing faces from real life!)
just keep on encouriging him to constanly improve himself, keep on checking your work with the examples in the tutorial, he should be fine! the talent is there, it's just a matter of shaping that talent! and since your son is only 13, it will be easier to shape it now, then in 5 years or so. (kids allways pick up stuff faster than adults..)
As long he has fun drawing, go for it!
and oh yeah: make sure he doesn't only concentrate on comics or manga, if you can do real life, all comic / manga stuff will be sooo easy... while he's still young, developing skills will be easier for him, and only concentrating on one aspect of drawing does not help..