That’s no problem, right?
He is not rejecting you!
He is simply saying that he can’t join you for a drink at that time.
Simply turn around and never speak to him again!
Of course, stay respectful and simply ad:
“No problem – maybe some other time…”
You can follow up with the conversation you were having or simply shift topics.
Remember that there is nothing to be embarrassed about.
It’s not the end of the world.
In fact, if you are well prepared you are already thinking about your next potential date for Saturday night.