Often you can catch him flatfooted at this time, if you're ready to follow up and keep gaining range. Most common of all is simply leaning away from your initial attack. Then you can know exactly what to do, since he has tipped his hand, and show his intention. The chin is the magic button. Tuck yours, exploit his.
Some people look really tough, but they go down from a tap on the chin.
Whereas, trying to knock a guy out by punching his skull can take a while, unless you hit really hard. Head's like a helmet. Not a good target, unless you can already break patio blocks with your fists. I've knocked people out by punching their skull without hurting my hands, but it takes a while to get your fists tough enough for it. Some people use the jab in a light way, like a fly swatter.