might be wrong with this thinking, but since he already told you he has one boy you just need to look at the probablity that the other is a boy. each kid had a 50% chance. so the remaining kid of unknown gender has a 50% chance of being a boy.
if he did not say one was a boy, it would be 1/2 * 1/2 = 1/4 or 25% that he had two boys
