He wasn't required to "defend" himself as the egger posed no threat after the egging.
Plus his reaction wasn't "split second", he waited moments then slapped the kid, the kid wasn't hurt so Anning attacked futher, the kid then had to defend himself from Anning, then a bunch of meathead attack the kid also.
The kid should have yelled "Gay Panic Defence" and "Rape!!!" when the Anning supporters mauled him.