Please can you post current test results for ammonia, nitrite and nitrate? Also what is your GH, KH pH and temperature?
It maybe looks like it's just pigmentation. Has he always been the same or has it developed recently?
His fins look a little ragged and maybe clamped so I would definitely recommend upping your water changes- 75% now then 50% every other day for a week or two. Incorporate gravel cleaning, filter rinsing (in tank water not tap water) and glass/ decor cleaning too.
If you do get a clearer picture please post it.
All the best.