How long is it appropriate to wait after a breakup before flirting with someone?

There's this really sweet boy I know that just got dumped pretty harshly by his girlfriend, apparently for a bunch of "flaws" that I actually find pretty attractive. (shyness, over-politeness, clingyness, etc) Anyway, I would like to make it known to him that I like him, and see if he likes me back, but he's probably still feeling bad and I don't want to rush things if it will make him uncomfortable. How long should I wait after the breakup before I get into serious flirtations?