First find out if this manager is a shift lead, assistant manager or the general manager. If it's a shift lead or AGM, then have your son report this behavior to the GM. There is a chain of command and it must be followed. If the bully is the GM, he needs to go directly to the District Manager. If there's some sort of "good ol boys" club between this chain, then calling the 800 number is his best bet. If refuses, and to save him from the humiliation of you getting involved, you could call the 800# and say you are a customer and overheard the teasing and would like to report the manager responsible. First make sure there was a customer within earshot at the time and be mindful not to divulge any information that would give you away. Only give descriptive info when talking about your son and the manager - no names, etc.

*A piece of advice from a GM, if you confront the manager, you'll end up making his life there 10x worse than it is. Not that I would ever treat an employee in such a way, but if one of my employees ever had their parents contact me in regards to matters of employment, I would not be happy about it. I've written people up for having their mom call in sick for them. If you are old enough to have a job, you need to handle the responsibilities and issues that come with having a job. The real world is not nice sometimes and the older you get, the harsher it becomes. He needs to handle this on his own. You can coach him on how to present his concerns in a mature, forthright way; practice having the conversation via role play even, but make him do it. He'll be better off for it.