How bad is it for a 15 year old to take sleep pills occasionly(when he is having a sleepless night)?

Not good: It would be much healthier and more effective to investigate why the sleep disturbance is present, than to simply medicate it. Then a more helpful intervention could be used. Depression and insomnia can happen at 15, but so can many other issues that deserve attention also.

Depends: A lot depends on why you need the pills. If your sleep is generally ok, and your doctor has cleared you to take the pills, then it should be ok. However, a thorough discussion with your doctor regarding stress, sleep disorders, proper sleep hygiene, and other possible psychiatric issues would be a good idea. Melatonin therapy may help, but often a good workup can find the issues with your sleep.