Starting a personal project is the best way to learn a language. Coding style, from mine experience with co-workers is a very subjective issue, ie why companies have strict code-styling documents, which any developer has to adhere to when working for that company. C, is a lovely language to learn coding, i think it the best as, it is not lengthy. It is beautiful, you can do, i think everything, if you wish. The learning curve is not steep, yes but implementation curve is very steep. And I think that the way to learn programming. Get the basics straight, and try to implement it. Put some time and effort into thinking, designing and then coding. And, when you are stuck, come here to SO or go to bing/google/colleagues.
If your motive is learning, try to implement something is already implemented. Learn the design pattern/styles, coding patters/styles. And evolve as you spend more and more time.
To answer your question, give him some nice challenges. If he knows the basic of C, ask him to implement (depending on his interest, say if networking is his interest) say a concurrent TCP server. Its not too difficult if you know how to make a TCP server, and you can think of the fork() trick. Or, if his level is higher, why not a threaded work-queue model. It will teach him locking/thread-pooling. And he can have a look into plethora of code on net, discuss with you/friends the solutions in terms of design and style.
It will be more fruitful to have a motive in head, like a task, and then look for a solution, rather than just following some code, trying to figure its requirements, which might in the end does not interest him domain wise. Like if he likes image processing, TCP stack's implementation wont interest him.
That is my recommendation.