that's bad. I'm assuming that, without the blurring portrayed in photo, he is covered in a bazillion tiny white dots.
it's a severe case, & the infection is worse inside their gills than shows outwardly.
knife fish are sensitive to meds and salt too, so you have to treat them at half strength. and that makes it harder to cure. ideally, you'd want to start treating knife fish before things get very bad.
make a thread in freshwater diseases section of forum here to get specific instructions. I'd recommend PMing Aquanero & ask for fast instructions. he should be able to get to you in morning. you can pm other mods in there too.