Hi - if he's read 1 and 2 he must be fairly advanced - unless you mean that he's read them in his own language? If so, and he's not an advanced learner, reading the whole of book three in English would be tough and unlikely to acheive much. Two possibilities :
If he's advanced :- Let him read the book in English at his own pace for homework. At the start of each lesson he summarises for you, in English what he's read. (speaking practice), and you use extracts from those chapters for detailed comprehension, structural development or vocabulary work - as seems necessary.
- Get hold of the DVD. Show him a scene from the section he's just read, but with no sound. Afterwards give him an "observation test" : What was there on the left of Professor Snape's desk? What was Hermione wearing? Who was Harry talking to when Dumbledore walked in? (more speaking practice)
- Then set some general comprehension questions : What does Harry want to do? Where is Hermione going? What's Ron's problem? and play the extract again with sound.(listening practice)
- After checking the answers give him a gapped transcript and ask him if he can predict the missing words. he then listens again to check his answers. (language development)
- For homework you could also get him to write about the books : Who is your favourite character and why? Which character do you like least and why? Would you like to go to a school like Hogwarts - why or why not? Imagine you are Harry and write about a typical day in your life at Hogwarts.
There are also lots of Harry Potter websites that you could look at with him.
If he's only intermediate level : - Tell him to get a copy of the book in his own language as well as in one English for you and to read his copy first. As he does so, each lesson he summarises for you, in English what happened in the chapters he just read (speaking practice). Help him express his ideas simply and feed in any necessary vocab - you'll need to have read the chapters yourself to be ready for the vocab which will come up.
- From the English versions, pick out some relatively simple passages from those chapters - keep them short and choose passages which use only the structures he's already learnt. Use them each lesson first for reading comprehension (his previous knowledge of the story will help get him over the language difficulties) and then vocabulary development.
- At this point you may also be able to use some extracts from the DVD as suggested above.