A kind of zombie inspired disease, without the loss of cognitive function and the eating of flesh. So, not really a zombie, just zombie inspired.
Poor thing. He looks happy though. Well, people enjoy themselves through all kinds of things. He's not that far advanced at this point anyway judging from how much color is still in his hair and such. Okay none of this scientifically makes sense, but it doesn't have to.
Later symptoms... inability to feel pain, lack of muscular inhibition, + others probably.