I didn't start wrestling until my sophomore year of high school. I ended up wrestling off someone who's been wrestling since 5th grade for a weight class and beat him. I got to be district runner up and district champ in back to back years although I couldn't get too far in regionals because my coach shafted me and forced me to wrestle two weight classes above what I should have been wrestling. I think I could have easily made state if I was at my normal weight class because the person I beat in the wrestle off made it.
...Soooooo no!! It's DEFINITELY not too late to start. Wrestling is all about determination and your will to learn. If your son has the will to do it, then by all means start. I have two other friends that started their freshmen year and one got 5th in state and another one got in the top 8 and he's still a junior (this guy was like...85 pounds as a freshman haha).
One thing I want to tell your son is this though. To get him started, please please get him some place that will extensively work on his stance and footwork. My high school coach did not spend a lot of time on stance and footwork and I suffered as a result. I finally learned better stance and footwork at a camp and my game improved by leaps and bounds. So to get started, find a place that will spend some time with the stance!