wow, that's pretty weird. from the picture it looks like he might have labored breathing? I really don't know what to tell you. Have you check all your parameters?
Done any water changes since his decline?
Whats the temperature of the water?
He looks in good shape other than his behavior I cant seem to notice any outward signs, so that's what makes me think internal which makes me go toward sudden change in water...
I was doing 25% every other day when I first noticed him acting strange, but then I started thinking that if he was just sulking then the water changes could stress him out even more? I'm currently doing 2x 1/3 water changes a week.
He does seem to have started breathing heavily over the last day or so..