Somewhere floating around on the net is an old newspaper interview (from before ESB's release) in which Prowse announced that Vader was Luke's father. But various sources have stated that Prowse wasn't privvy to this information during filming. So, was it just a good guess?
Anyway, Prowse was indeed blamed for a leak and side-lined from many important scenes in ROTJ (which made use of a stand-in in the Vader suit).