When does Scott Bradley Jr. enter the Kell Hounds, and when is he made Leutenant Commander of the 2nd Regiment in substitution of his father?
There doesn't appear to be a conclusive answer I'm afraid, but FM:Mercs Revised p80 has the following -
"Lieutenant Colonel Scott Bradley looks, sounds and acts so much like his father, the late Scott Bradley Sr., that most public records confuse when the elder Bradley retired and the son stepped in."
It goes on to state that Scott Jr is 40, and the in-universe date of the book is 3067 (from p6).
Hope that helps,
Chris W