He was the executive producer on TWOK which, oddly enough, despite the word "executive" is "outranked" by the producer. Robert Sallin was the producer.
That's a little misleading. As producer, Sallin had a more hands-on job and was reponsible for a lot of the decisions made for the movie, but as Executive Producer, Harve Bennett was his boss. I would guess that Bennett got more involved in Star Trek III because he wrote it too.