I imagine a lot of what we know about him and his movement was written by his enemies so it's hard to pin down what he believed as a lot of what the wikipedia article claims sounds too good to be true. The idea that a major religious reformer challenging social inequality would find the favour of an emperor sounds bizarre; did this actually happen and what led up to it? And did the memories of the movement have a longer lasting influence on thought in the area?
Somebody else may have more to add, but I've addressed Mazdak, his roots/legacy, and his relationship with Shah Kavad I in this older answer.