We told our daughter that we wanted to help, but we were unable to understand her when she tried to talk while crying. Even if she was bleeding or something incredibly obvious that we knew was bothering her, we told her we couldn't understand her while she was screaming/crying. Then we'd get her to take some deep breaths and have her explain the issue in calm words. Once she got to that point, her lizard brain was no longer in control and she was able to understand that her problem was inconsequential, making it much easier to handle.

Also... don't fear for his Kindergarten experience. Only one kid in that teacher's wealth of experience will be the biggest whiner. Super low chance it's your kid. And if that teacher was able to cope with the biggest whiner ever, your kid will be much easier. If it IS your kid, he's only a little worse than the worst ever and they handled that other kid just fine.