Assuming no air resistance, using a the simple mechanics equation of

s = ut + 0.5at^2

You can derive that the boy falls 3km+

Well even if there is air resistance, the final distance the boy falls is 929m

So assuming that the boy indeed falls 929m, superman starts flying at 18s. So he takes a total of 10s approx to travel 929m. Works out to be 334440 km/hr. Definitely faster than a speeding bullet :)