Anyone applying through the express entry program can only get points for a maximum of three years for work experience. So if it is difficult to get a letter from your previous employer, and your current employment matches your selected NOC, then I would just forget the previous employment you had. And only declare it as an employment in the personal history section during your application.
However, if you still want to include your previous employment, then search in Google about, 'employer refusing to provide reference letter express entry'
There are plenty of both videos and blog articles of how to go about.