First your kid must learn how to get on. This is shown at 0:45 in this tutorial https://www.youtube.com/watch?v=Ks9yLrXjKgU
Explain that once he is on, if he puts all his weight on the pedal in the 6 O’clock position, the uni will not move and he will not fall off.
Then you hold his hand and walk beside him until he his comfortable with that. This phase can take a few days and requires some patience on your part, because he will probably get frustrated and yell at you at some point. You may find this phase a little boring also.
Then pull out a chair, a beer, and the garbage bin. The chair and the beer is for you, and the garbage bin is to support your kid so he can mount his unicycle by himself.
He has to get in the positon where he has the pedals parallell to the ground. Then lean forward, let go of the bin and just try to pedal away. This phase can take from a few hours up to a couple of weeks (or maybe more).
There are a few threads on this subject, for example:
http://www.unicyclist.com/forums/showthread.php?s=&threadid=21020
http://www.unicyclist.com/forums/showthread.php?s=&threadid=21059