Congrats to your son on the interest! You said he has been emailing this particular coach so it shouldn't be anything to get stressed about since they already have dialog. I would suggest he have a few new questions ready about the school or program and be prepared to answer questions like where he is in the process, what other schools he is talking to, if he plans on making a decision soon, etc.
As long as he is respectful and himself and shows interest in the program, he'll be fine.
You said...
Coach asked son when a good time would be to talk and son said he would get back to him. Since his flight did not leave until later this evening, I suggested to son he should offer some time to talk today, but son said, "I am sure tomorrow will be fine."
I am probably reading too much into this but it almost sounds as if son is thinking he dictates when he will talk to the coach. Even though the coach made the courteous comment, I would, instead, suggest that son offers multiple timeframes and shows flexibility to make himself available when it is convenient for the coach.