Well, I haven't met his mom, but his dad is pretty short, so I don't believe he has much heighth to gain and would agree that he might be done growing.
I can't agree more with what you said about girls not being better as a senior vs a freshman. I know many girls that performed their best their junior year and regressed as seniors in both basketball and track.
One example-My coworker knew Tiana Reeves (sp?) of Gothenburg in junior high and knew she would dominate in high school, which she did (she won state long jump and triple jump by more than a foot multuple years 2014 & 2015 I believe) but her impressive numbers dropped her senior year even. It happens all too often in high school sports.
I would imagine Dylan will improve his times, but not substantially as I agree he is probably done growing. I know his dad's old hometown was proud and were paying attention and getting updates throughout the track season on Facebook.