Tradesman services and terms of engagement?

My partner is an industrial electrician. He has many regular clients. The thing is that when he gets new clients, in the past he has traveled to see them to look at the problem, spent up to an hour essing if he can do the work and then told them that it is a mechanical problem and referred them on. He tries to get as much info over the phone from them before he goes to see them to es the problem and insists on COD for the first three times he uses them, however some of them think that they don't need to pay him for such a short time. They also know that it is not worth him chasing the money because it will cost more to recover it than what the job is worth. We are trying to work out a new system whereby he lets them know over the phone that there is a minimum call out fee payable on the visit. He is a sole trader and works on his own, so there is no fallback. How can he enforce this new rule without being rude, and what do other tradesmen do?