[Siddhartha]

Getting to the point where you have truly internalized language bits and skills so that you can employ them as necessary in a natural, non self-conscious way takes extensive and ongoing practice.

The long and the short of it is, he needs a lot more practice conversing in the contexts in which he wants to perform well. Also, the more similar his practice sessions with you are to the situations he finds himself in with his clients, the better.

What I would suggest doing is:

1. Talk to him at length to find out as much as you can about what specific topics he has to discuss with his clients, what things he is usually asked and what kinds of responses he must give, etc.

2. Use this detailed knowledge to custom tailor various role-play situations where the two of you practice doing exactly what he must do during his work.