I Believe he started experiment with testosterone in about 55- 56
Testosterone was available even from late 40's
that video above was shot in 56...
In the year 1957 dianabol was invented and available, especially for famous people like Steve Reeves. Steve was friends with Reg Park. I suspect Steve and Reg started experimenting with dbol that year, 1957. Steve Reeves probably quit it and focused on acting because he seemed to get smaller after that time.
These pics are from 1957:


Reg Park 1957:
