Just time your blocks. There's a certain rhythm to it. Once you block it five times, he'll get dizzy and you can punch him for a one-hit KO.
Are you emulating or playing on an HDTV by any chance? If you're having difficulty getting the timing down, it may be an issue of input and/or display lag. This game is notorious for giving players an unnaturally hard time if input lag is present, since so many attack patterns (especially in the later fights) and counter-attacks come down to timing your blocks, dodges, and punches with very precise timing. Only rhythm games suffer more.
