Ever since I've been doing Zombie Roomie, I'd occasionally get asked, "Does George smell bad?" "Why doesn't George rot away like traditional zombies?" and other similar questions...
This strip explains it... as long as he's maintaining his diet of fresh human flesh... he can stop the decomposition and the side effects.
John didn't go kill some random guy by the way. As we've seen in the old comic "Eye of the Tiger"
, John has broken into the morgue at least a few times with George to get bodies. We can safely assume that John got some bits and pieces from some cadaver there to nurse George back to "health".