From what I understand *spoilers* he didn't actually spend 5 years as a zombie, but rather spent 5 years hibernating after he was buried because he was so hurt he needed all that time to recover. Then, relatively recently he woke up and joined the Zombies, so I seriously doubt he even had a chance to even try to eat a human brain.

And he is such a Gentleman he even forgot he needed to eat after those 5 years. =P