Hi
Is it a sore on his mouth area? My dwarf gourami has a reddish sore on his mouth so I took a video on my phone and showed it to my local fish expert. He said it is a fungal infection and it affects gouramis more as the have bigger sort of lips and because they go up to the surface more this makes them more prone to this.
I got Interpet Anti-Fungus and Finrot. The way he has cured fish who have had it is to give 15mls on day one 10mls day 2 and 5mls day 3, on day 4 leave and see if improvement if not repeat it. Also make sure your filter is working well as it needs good oxygenation for this medicine aparantly.
He only got first treatment today so I am hoping he will be ok. I did a
water change before it. All my other fish are grand though.
Hope your fish gets better
Liz