The Shabbat before a groom gets married, there is a custom to throw candies when he is called to the Torah reading. This is to wish him a sweet new life as a married man.

In the last 50 years or so, in the US and some other Western countries, some have extended the custom to a Bar Mitzva boy as well; so as to add to the festive atmosphere, and that his "new life' as obligated in mitzvot should be sweet.

However, there is no written source (to my knowledge) for the candy throwing custom at a Bar Mitzva.