Sounds good to me! I used to be a sprinter during the summer when rugby wasn't taught when I was at school, 200m sprints, but still have friends who run long distance. Year 11's (not sure on that age 15-16 I think) with some talent can run 1500m in around 6.00mins, so doubling, and adding a bit of time for fatigue... I'd say your son is on the right track, you should take him to an athletics club get him properly tested! they'd probably be more helpful then me just saying yes he is!

http://www.uka.org.uk/

have a look on the UK athletics website for local clubs, (presuming you are form the UK, if not look for your local NGB)

Alex
Rugby Union & Sevens Player