Hmmm... he was landing punches on Cain in the exchanges and they both demonstrated poor technique, and Brock undoubtedly hits with force. IMO it was his poor hit reaction that let Cain get the upper hand, not being outboxed.
Point being, even the best strikers get hit, but none of them would win fights if they reacted like Brock does, so I don't think any amount of strike specific training will help him out against even average strikers. Even if Brock were a good striker, he wouldn't be a slick one, he'd be a hard hitter who can take a lot of damage.
Brock should just train his takedowns, train to use his bulk as a guard, close the distance and clinch, which is a position where he should be able to decimate people because as soon as he takes a shot he doesn't like, he can just trip and takedown, but he doesn't know enough about distance and footwork yet.