I have a Female Thai Silk who had almost the same issue. I too tried Parasite Guard with no effect.
I stumbled upon this video on Facebook and it cured my Thai Silk after doing as the video says!
[video=youtube;0sw8H9KvEf0]https://www.youtube.com/watch?v=0sw8H9KvEf0&list=LLQF7UB_BWtLCxVSZlktW7lQ& index=2[/video]
Shes healthy again and fully eating after almost 3 weeks of rejecting it.
I hope this helps / works and he can be healthy fish.