Ask him why he thinks it's okay for him to beat on his younger, smaller brother, but when you use necessary force to get him off of his brother, it's "abuse".
Also, point out to him that it's not abuse. Abuse is when you take it too far. You protected one of your kids; the fact that you had to act against another one of your kids doesn't really matter. So he bumped his head. Big fucking deal. Tell him to man up. If he's gonna beat on someone younger and smaller than him, maybe he ought to know what it feels like.
Note: You didn't beat on him; you didn't even overreact, as far as I'm concerned. If he wasn't beating on his brother in the first place, no one would've gotten hurt.