You just need to let him know you expect him to cum by servicing him during your session and if that would be a problem. If he says no problem, I would not worry about the number of clients. As long as they provide me the expected service, it does not really matter to me. I have talked with my guys before. Some have had one or two before me and sometimes more. If I just feel like being serviced that day, I tell them they can save their cum for another. Otherwise, they will cum for me irregardless.