Can you get a hygrometer to measure humidity?

What does his poop look like?

Get some clear, unflavored, plain Pedialyte. Mix it in a 1:10 ratio of pedialyte to dechlorinated water, spring water, or distilled water. You can either soak him in it for 15 minutes or do slow gentle drops of it on his back. This is just to give him some electrolytes.

A good thing to do is have a poop sample send for testing. Ova, parasites, culture.

Are you using calcium with vitamin D3 also? If not, you need to. It should be lightly dusted on his crickets every other day, except on vita day. Symptoms of low calcium levels: numbness, tingling, twitching, tremors, uncoordinated movements, lack of movements, seizures, paralysis, lethargy, also lack of muscle functions such as smooth muscle organs - intestinal peristalsis and constipation, and cardiac muscle functions if severe. Vitamin D3 is required in the supplement for proper absorption of the calcium. Lack of calcium leads to these problems and can cause Metabolic Bone Disease. Good supplements are Repcal calcium with vitamin D3. Flickers sells a liquid supplement, I think Repaahy does also. I like powdered supplements for routine care. I have the liquid on hand for emergencies. You can do a dilute soak with the liquid.

I'm not sure if its just a nutritional deficiency or if he may have a parasite? Or other?

Panacur is a good parasite treatment. It's dosed by a vet per weight/size of the frog.

Perhaps you could send a stool sample for testing to a vet, in case it's a parasite?

Are you in the states?