I’ve noticed that my dad has had the same semi-receded hair line for like the last 15 years. It has only slightly thinned out on the top since.
We are born with a patterned hair loss (inherited from family) that we will reach and then stabilize. That is most likely why your dad’s patterned hair loss is stable. Let’s stay that he had a Norwood 4A pattern genetically programmed, that means he, most likely, will never progress further to a Norwood Class 6 or 7 pattern.