He apparently started in the early 1980s (if not sooner), since his first book on the subject was published in 1985. Here's a blurb from his 1989 book:
"Exploring the World of Lucid Dreaming goes far beyond the confines of pop dream psychology, establishing a scientifically researched framework for using lucid dreaming. Based on Dr. Stephen LaBerge’s extensive laboratory work at Stanford University mapping mind/body relationships during the dream state, as well as the teachings of Tibetan dream yogis and the work of other scientists, including German psychologist Paul Tholey…"https://www.amazon.com/dp/034537410X
"Exploring the World of Lucid Dreaming goes far beyond the confines of pop dream psychology, establishing a scientifically researched framework for using lucid dreaming. Based on Dr. Stephen LaBerge’s extensive laboratory work at Stanford University mapping mind/body relationships during the dream state, as well as the teachings of Tibetan dream yogis and the work of other scientists, including German psychologist Paul Tholey…" https://www.amazon.com/dp/034537410X