lucid dreaming sleep is not as deep and refreshing as "dead" sleep. Especially if it causes you to wake up multiple times each night... It is ok if you have the luxury of sleeping long and after that all you have to do is sit in school or in an office... But it gotta be hard for someone who does exhausing physical training.

As for not lucidly dreaming - just let go, drift off, lose awareness, let the dream overcome you. Or try to fall asleep in a dream. If he's truly lucid every nigt it wont take him long to figure out a way to lose his lucidity...