Unless you concerned about hurting the guys feelings. . . Or if humiliation and punishment are part of his limits . . . Unless you need to be careful enough with the guy so that he comes back in a few weeks or a month,

Then, you explain or act a bit angry as he has broken a rule you have, and he will be and should be humiliated and punished.

The guy should eat the come, be caned or whipped for an extra 40 minutes or for the rest of the session and told that for other visits with you, he may be expected to payan extra fee for the session.

Tell him that he is now set for cum eating instruction for some of the next few sessions,
And or you will be smearing his face with the cum, and a bit angry with him.

At least, that is my series of suggestions and one know at least one domme who does most of these things when the sub comes early.