Can i give my 21 month old son 0.5mg of melatonin for just one night? is it safe?

NO WAY: Under no circumstance would I give a 21 month old baby melatonin. He needs to learn how to console himself and fall asleep on his own in order to become a good sleeper. Lights down, no tv screens, quiet bath, books, then bed. The hardest part as a parent comes after this. Ignoring. Working now to help him develop good sleep habits will pay off later in spades!