It sounds like you're doing everything right.
He should be drinking.
Maybe there's an underlying issue somewhere, illness maybe? Is he healthy apart from dehydration?
I have both an analogue and a digital hygrometer.
My analogue one is similar to yours, whereby it stays at around 70-80%. Whereas my digital one seems more accurate with readings from 55-80%.
Get yourself a digital one.
I would say the only limiting factor is his enclosure now.
Adult male minimum should be 60 x 60 x 120.
And to be honest, you'd probably need an even larger one than that to reach 24".
It looks like he can get really close to the heat lamp with those branches.
You need to lower them and ideally have a basking branch which is horizontal and about 6-8" from the top of the screen to stop him getting burned.