I don't want to change your words, and in fact you shouldn't have to change your words. It's your reader's responsibility to keep track of the action and who is speaking, but of course it's YOUR responsibility to write narrative that is distinctive, well-written, and unambiguous in its important points.
In the paragraph above, unless you are deliberately trying to confuse, it's obvious who is meant by "he" in each case, as the paragraph is fairly well-written. However, there are ways around the dilemma, if you wish to make your action more clear:
He saw a man in the atrium, standing in a group of other men, wearing a red sweatsuit. He figured that was his mark, but he wanted to make sure. It's not that you should copy the crappy bit of action that I am putting out here--I sincerely hope that you won't--but there are ways around the conundrum you're having. Those ways involve good cohesive writing.
As he made his way down to the crowded atrium, a waiter intervened. "Drink, sir?" he asked in a chilly professional tone.
"No thanks," he replied, and continued down the main staircase into the atrium.
A reporter he knew, whose name he could not remember, appeared suddenly, asking questions. "I wasn't there at the time," he answered absently. "Please call my secretary."
Now the man in the red sweatsuit was disappearing into an alcove by the potted palm. He had to get down there before his mark disappeared!
I hope this was helpful.