Try googling \"fat naked\" and look at the pictures - I dare you
I´m obviously NOT gonna post pictures here *shudders*
My point is that when you get very fat - and moist as this guy surely is - the skin suffers badly in all wrinkles and creases - it starts to get irritated, sore, inflamed and such. I can only imagine that a guy who is called \"the Great Unclean One\" would suffer from a lot of this.
So you need to treat all these areas of the skin with some kind of inflamed putrid effect/color to make it really disgusting.
Edit: remembered that adding some necrotic skinpatches might also be an idea - the skin on VERY fat people has a risk of just dying because the vascular system can´t keep so big a mass oxygenated (you can do a goggle search on that one too - if you´ve got the stomach for it
).
Lastly maybe darkening the inside of the sores so that they go almost black in the recesses (you can use tamya clear red and black ink for this: http://www.bolterandchainsword.com/index.php?autocom=ineo&showarticle=298)