One of the most important things to remember, too, is that however hard you kick him there, the physics of the area make it so he won't be feeling the pain for a good 30-40 second. Don't ask me why, this is just the way it works. However, he knows that in a few seconds he's going to writhing in pain, and this knowledge is usually enough to make him go berserk in the mean time.
He now has license to go completely insane.
You have just done to him one of the most painful things anyone can do. Everything else you do afterward will be, therefore, mild in comparison. And so he has nothing to lose at this point.
Use with caution, for the love of God, use with caution.