All the training videos that I have seen brock in involved strength training and sparring, but the sparring was one sided in that Brock was the only one throwing punches. He should do defensive drills where he can't strike back but must defend against a flurry attack.
No one expects him to strike like Overeem, but people do expect that if you are a fighter you are use to taking a punch or two. I like Brock (he has warmed on me), but I think if he wants to get to the top then he needs to work on his holes.
"Let them see that their words can cut you, and you'll never be free of the mockery. If they want to give you a name, take it, make it your own. Then they can't hurt you with it anymore." - Tyrion Lannister