Also, it's been my experience that flipping turns on laps can more quickly exhaust an inexperienced swimmer who doesn't know how to pace/breath through the turn. They are frequently expending way too much energy and holding too much breath (and wasting time). There are lots of great exercises and easy tips that most coaches can share in 5 minutes that your son can run drills on. This alone, may help him.
The other thing for endurance: ocean/live water swim mixed in with pool training.
lastly, on the 1000M specifically... there are great tri-athlete training programs (found all over the web) that are pretty easy to follow and give you a day/day breakdown and will virtually guarantee the endurance build he may need. This is how i made it through training for my first mini-tri with only a 8-week ramp (and believe me, although I used to swim competitively I WAS NOT IN SHAPE!) I'd also be happy to email you the 3 that I've got (range from mini to full tris) tht were given to me by guys who are hard-core triathletes.
good luck to your son! He'll probably get through just fine anyway. Aah, to be 22 again!
:) m