Metronidazole and Panacure are not antibiotics, they are anti flagellate/parasite treatments.
I wish I had answers for you, have they tested your beardie for atadnovirus (spelling?) ?
Have you contacted the breeder and asked them what could possibly be going on? I wonder if there are some underlying genetic issues here, seeing as its a partial leatherback?
I have a veterinarian who specializes in beardies, (she even goes to Australia every year and studies them in the wild) I wonder if your vet could contact her for some help/information?
Is he getting UVB by chance? if so, what kind/how much?
Help! Crested Geckos! 0.1.Leachie, 2.1.1 Gargs, 1.0 Chahoua, 2.1. Blue Tongue Skinks, 1.1. White Lined Geckos, 1.3. African Fat Tails, 2.2.0 Tokay Geckos, 1.0. Giant Day Gecko, 2 tarantulas