Perhaps some larger wrinkles around the belly? In Zbrush just start sculpting some stuff on a new layer, tracing around where wrinkles / folds might be and see if the result looks acceptable. Perhaps even a scar or two (making parts of the diffuse slightly brighter/whiter yellow, or reddish if they're new scars), or a…
You might consider adding a little hue. saturation variation to the skin, perhaps a little countershading to break up a bit of the very monochrome skin. also adding some warmer pinkish tones in the face around the nose, cheeks and mouth, and some darker less saturated tones around the eyes could help make him feel more…