People often feel guilty about charging for passing of knowledge, and don't realize that by not assessing themselves accurately they not only cheat themselves, they actually deprive their clients of the satisfaction of having got what they paid for. By selling himself short he might seem less than he is for others.

Ask him to evaluate his time regardless of the customers. He should factor in his years of learning, his experience, his level of expertise. Ask him how much he think he would pay for an hour with a person at his level. Then factor in the variables of traveling, electricity and anything else relevant to the actual lesson.

Add 25% profit, and there you have an estimated fair compensation for his time.