Students and the companies they work for often have unrealistic expectations about what can be achieved in a short space of time, and unfortunately, schools often promise that they will meet these unrealistic expectations, in order to get the contract with the company.
The responsibility is then of course passed to the teacher, often with little support from the school about how to achieve this.
It sounds like you are in a situation a little like this. So, what can you do? Like you said, you want to do the best that you can, and this is exactly right. What you can do is teach your student to the best of your ability. If your employer has indeed set unrealistic goals, then, as long as you as a teacher have done everything you can in the time you have, it is your employer's responsibility if these are not met, not yours.
From what you've said, it sounds like your employer has not done a proper analysis of the student's needs or objectives. What exactly does he need to be able to do at the end of 12 weeks that he can't do now? This is something that you can ask him directly. I know you've probably had the first lesson by now, but the next time you see him, I would do what your employer didn't do, and ask him what exactly his company expects him to achieve. Does he need to be able to converse more effectively with clients? Write business emails? Read and summarise reports? Make presentations? etc etc.
This will give you a clear set of objectives to work towards. Then, be realistic - depending on the time you have with him each week, and on how much work he can do outside the classroom (ask him this as well), set objectives for each week.
If his objective is simply to improve his grammar, you will need to find out where his weaknesses are, both in writing and speaking. Your employer may have a test that they use to place students in different levels - these are normally grammar based and would give you a good idea of specific areas of weakness. Then you can do the same through targeted conversation - ask many questions in different tenses and using different grammatical structures, and see if he responds correctly.
I hope this helps. If you can let me know more specifically what the student needs and how often you meet him, I could offer some more specific pointers.