Looks like an ulcerating wound to me, not sure.
But what else can I see: all those tiny white spots all over his body and fins? Or am I seeing things?
TEST YOUR WATER ASAP, post the numerical results.
Make a waterchange accordingly. Add conditioner and beneficial bacteria.
Treat with Pimafix, Melafix according to bottle instructions ASAP.
You also may need some stronger medication, like antibiotics and medicated food to get rid of ICK ( this is what the white spots look like to me )
Minde has got a good treatment plan with epsom salts and rising the temp. She sure will be on soon and give you advice too.
just for a basic rule to remember:
ALL ILLNESSES COME MOST LIKELY FROM A BAD WATER QUALITY