So by this metric, it would seem that your boss would need to keep his license active through when you are applying.
Additionally,
@Funnpun, who will be in responsible charge of your work after your direct supervisor retires? I would think you would want that person to be one of your references, too.
Theoretically, depending on your work experience, your references should be in responsible charge over the time frame that is the experience you are listing on your application. Of course, to get to the four references you need, some of them may not.
I hope this makes sense!