Stutter is a complex disorder and there could be 100s of psychological, neurological, physiological reasons why people stutter. Kids sometimes develop stutter just because their vocal organs are growing so fast. Most grow out of it, as it seems to be in your dad's case. Ive been stuttering for 40 years, and have read perhaps every publication on stuttering, worked with 15+ different therpists, and stuttering program. I used to wake up everyday ashamed of myself, why can't i speak like everyone else, and beat myself to death every time i stuttered. ive probably experimented with virtually every speech technique out in the world by now. What helped me the most is accepting my stutter. With it came the realizition that i don't need to fight my stutter for another 40 years. NOW Im doing very well professionally, socially, and personally, but i haven't overcome my stutter yet. I'm simply more confident. Ive realized that most people dont give a shit if you stutter or not. So most adult stutters have very small chance of overcoming it completely, and for most of us, acceptance is the best way. Accepting one's stutter allows us to be more confident in our daily lives and show our true selves, instead of posing as a fluent person. it removes the anxiety, fear and stigma in our brain regarding our own stutter.