He earned the commission back in late Nov. when etc.
He should write to HR (cc'ing the old boss) specifically asking them when the November commission will be paid and by what date. If they say they won't do it then get a specific reason why not (they may actually be on legal ground after all, for example its possible the old boss didn't have authority to make the promises he made).
Once he has answers to those questions then he can consider what to do.
If they never respond then follow up again and ask for a response of some sort within another 2-3 weeks. If they still don't respond write a third time stating he will consider his legal options if a response is not received within a month.
But he has to be very specific about by what date a response is expected (a couple of weeks is probably enough) and also about exactly what info he needs. Otherwise he could very well be ignored.