Before that, he'd worked for a lot of people over the years. Before that, he worked for a lot of people over the years. Chris is correct. Also, saying "before that" repeats the past perfect tense of the first sentence. So for conversational English, you can use either. In fact, Americans often do. I think we're a little more casual than other English speaking countries. :-)

The first sentence is the right one. Before that he had worked. The past perfect allows us to refer to a past moment that is even earlier than the first action. Before action 1 he had done action2.