I have this a lot. I try my best to organise structured groundschool, give extensive breifings on naviagation, and yet students often do not put in the work at home.
What is the background of this student? I find the worst for putting in the work at home are the young 16-20 year olds who have a lot of other studies going on in their lives and are being funded by mum and dad. Since they are not paying for it they have no real incentive to put the work in at home.
The best for putting the work in seem to be the 30-40 year olds who have saved for ages and are finally in a position to pay for their own licence. They don't take anything for granted, and get the work done when you ask it be done.
The other one to watch out for are the 40-50 year olds who have been very successful in business and think that money will solve any problem. They tend to turn up very unprepared and never put the ground studies in.
With this student, you could say no flying until x number of exams are done, but then he may well take his business elsewhere. Perhaps if you just set a firm deadline he will cram at home and get the pass. Half the problem is that there is no fixed exam date, the student can take the paper when they feel ready, and often they never feel all that ready.