Lol. He is 13 not 3. At this age, most kids only think about the "best" in everything. Don't pull his leg
First of all, what is your definition of a "heavy bike"? if you mean a bike with a bigger engine, there are several categories. Sports, Street, Cruiser, super sports, off road etc.
For a newbie, the most forgiving bike is the best bike. If bikes of the same category but different manufacturers are compared, only an experienced rider, who can milk the top end performance, can tell the difference most of the time.
in this budget, one can buy a used 600cc (2006-2010) bike in good condition.