Stringy poo and a sunken stomach could also mean a bad diet.
Bloating isn't the same thing, although both 'could' be caused by a parasite. You've already started treatment, so may aswell finish the first course. I'm not familiar with Cloat? but if it's a bloat med, and you have a sunken stomach, it may be targeting the wrong parasite? (If it is). I've treated bloat with simple clean water in my QT, no meds. It was caught early. You also said in the opening question, you believe but aren't sure he is sick. Bloating that I've dealt with is very noticeable. The fish will rest a lot more than usual, like yours seems to do.
How long did you have him prior to this?
And a simple question we haven't asked yet
Whats the Ph, Ammonia, Nitrite and nitraTE readings? and hardness if you have the test for it.