Thanks! Working on getting the humidity up currently and we will try to soak him with some warm water. Do you have any recommendations on which brands are best to dust his food with?It sounds like he's looked after pretty good other than the small adjustments karieree suggested, if he's been eating mealworms as a staple I'd envisage him to be a bit fatter as they are very fatty bugs, though this can vary depending on feeding schedule too. He's still juvenile so it's probably best to cut down the mealworms to a few once a week only, they aren't very nutritious. When you're able go ahead and swap over to roaches or BSFL. I think at his age 3-4x a week on the roach/BSFL with dusting would be good as he's still growing. This could be a possible reason as how he's become unwell as he needs to eat frequently to keep up with his growth, the sudden stop can cause some damage to the liver sometimes if they otherwise had a healthy appetite (my vet told me this when my girl stopped eating at 18mo old but she bounced back within a week or so with supplement feeding)
The humidity being 10% is also definitely a possible factor, he could be very dehydrated- and he does look a little so. I'd try to get that humidity up a little and give him gentle warm soaks where possible (please make sure the room is warm too so it doesn't shock him!) - offer him some water on the nose as well and see if he responds !
It's good you're taking him to the vet tomorrow, if none of the above helps the vet can certainly help rule out acute illness. I'm rooting for your boy to recover !