Do you mean he can average 7:30-8:00 for 3 mile runs, or that he can run a 7:30-8:00 mile?
At 13 years old, running a 22-24 minute 3 mile off of no training shows he has some potential, but not necessarily varsity before he's an upperclassman (obviously, that depends on the size of school and caliber of coaching he'd get).
For a measure of comparison, we had two kids that could run 21 minute 5k's as incoming freshmen last year, and the only training they did was 8 weeks of middle school track (short season around here, or maybe that's the standard, I don't knot). Unfortunately, they also skipped out on most of the summer training, so I had to be pretty careful bringing them along this fall. But by the end of the season, one was our #4/5 (helping us to a 7th place finish as our #5 at state with a 17:36.8, and that's what I would call an "average" 5k course - neither notably fast or slow) and the other has run 11:20 for 3200m this spring - not varsity, but will probably make the cut for the league championship meet as a freshman (mid-sized schools). He'll probably be varsity either next year or the year after.
And as for the question of whether or not most 13 year old kids can run 3 miles non-stop... I'd say absolutely not.