First, there is regression to the mean, so his expected athletic level is below yours (although the variance is large).
As for the question you asked, I fit that scenario. In high school I start out at 4'11"/85 pounds and did not have much size or strength and this was a long time ago so that there were not a lot of people running. I improved steadily but slowly through HS but still finished usually just barely making the JV top 7 (it was a big HS - 3100). Right at the end I got better (finished 5th in league in JV race, beating many people I had lost to earlier in the year). I also ran a very good 600 in the spring in a PE test that indicated that my growth (ended high school at 5'10+/142).
I ran sporadically and road a bike for transport and exercise through to part way through grad school. I then started running regularly and a lot (ran with a DIII team for a number of workouts, although I was slower, especially at first). My objective was the marathon, so my marks at shorter distances were not all that good (e.g., 51 for 15km), but ran mid 2:30s for the marathon and might have hit my 2:30 target except for a plantar fascia problem that ended my running adequate mileage and I raced on the bike fairly successfully for three years.
Now 2:35 is not a great marathon but it is much better than most marginal high school JC XC runners get to.