Hi Mike,
WIth Panacure you will have a much better success using it as an oral suspension.
Before you treat him, are you feeding him earthworms? If so, the worms have a high nematode population that can cause false positives as to the type and severity of the infection.
Also at least in herbivorous herps (which removes caudata here but still may be of value), some nematodes are commensuals and help with the digestion of fiberous food items.
I have seen weight loss in Salamandra kept on acidic substrates (personal observations)as the stress apparently prevented them from feeding as well as they normally would.
Ed