Think of it this way, if he had hired you to be the pilot (not instructor), he would have to send you to the school anyway and then PAY YOU. Many aircraft manufacturers include one or two of these classes with the initial purchase, so he may have paid nothing for it.
If, as you say, he will need +/-100 hours of flight time, I do believe you would be smart in giving him a reasonable discount (10-20%) since he will be giving you so much business.
I would also be weary of him doing these cross countries. He may be thinking he will "let" you give him instruction to and from a business meeting. Which would leave you sitting in the FBO for several hours. If he pulls this, I would suggest a flat day rate: for an eight hour day, I would probably charge him for about six hours (think of it as a 75% duty rig). An alternative is to charge him a fee for waiting.... just like cabbies do.
This guy is probably one of the folks who doesn't bat an eye at paying $100 per hour to get his BMW fixed, but will moan all day about his A&P charging $50 per hour. Be careful.
Lastly, be sure either the flight school or his insurance covers YOUR actions. It would really suck for you if he runs the thing into a ditch while you are instructing, and he stick YOU with the bill.
Be VERY careful.