Can I suggest that he writes up some paragraphs related to his teaching matter and gives you mini-presentations. This is rather dull rote learning but it's also pretty effective for getting the subconcious to absorb things.
He needs to read avidly in English in his area of expertise.
Have him teach you his subject so you understand it and do specific exercises to address specific grammatical weaknesses which emerge.
If he loves his subject he should find it interesting telling you about it.
If the idea is to get him to practise talking as much as possible, talk about the things he is interested in rather than bringing in some article to discuss about global warming (or whatever), which he might not give a fig about!
Fun English Games