I think in juniors riding a fixed gear is a very good idea. If you have a look at the Aussie pro sprinters at the moment (McEwen, Brown, Cook Ogrady etc) they all raced track. Junior sprinting is all about who can spin the fastest and for the longest. A fixed gear will teach this, especially if motorpaced as there is no rest. If his legs tire he still has to spin.

On a non fixed note. If he can find a circuit of say 3k with a couple of climbs (nothing severe) he could do hill sprints (100m or so) in his maximum gear. I found that doing 4 or so of these laps a couple of times a week really helped my kick but also my strength over the last part of the sprint.

Basically if you want ot get better at sprinting you've got to practice your sprinting. Natural ability helps but in juniors with the proper training everyone should be pretty close.

Riding track doesn't help with your leg speed, the gear you select for either road or tack determines this. Curly hasn't access to a velodrome but motorpacing on the road might be possible.

Motorpacing on a small gear does train you nervous system to spin and over time you will build muscle endurance at higher cadences.

When you got the time use 5mins of rest or more. As Zakeen said you have want to train your alactic anaerobic system. I have seen test results of sprint training were trained people had 3 minutes of rest after 10s of sprinting and their lactate was slowly increasing during the training ( > 6mmol/l) and that is something you don't want at a sprint training.