I am not a tropical fish bet but I would suggest the following, assuming your diagnosis.
I would not repeat the levamisole since you have used it three times already. I would suggest Fenbendazole. For three days.
Each day before dosing I would do at lease 80% water change and make sure you have removed all debris and mechanical and chemical filtration.
Assume your little guy will not have an appetite until days after the treatment. After treatment would raise temp to 90 degrees and feed live black worms. You might also use Enson salts during the treatment to help purge the intestines