It would be a good idea to pick up a
test kit.
If the parameters are off, the water can cause all kinds of problems.
Being the only fish doesn't make the tank size correct, he should be in at least 5g's.
How often are you changing the water?
Is the tank heated? Filtered?
Are you giving freeze dried blood worms or frozen?
Sometimes flakes and freeze dried foods can cause bloat.
For now, it would be a good idea to fast a day or two then offer him just a couple bits of pea on the 3rd day. Continue that, see if it helps.
Try re-sizing the pic. It might be too big.