yeah, it seems Revolyt is the one, i tried to google it but it came back not in English but go ahead if it is unflavoured and without any dyes, it should be clear solution, like water.
instead of dubia i would better do piece of nightcrawler coated with reptiboost, nightcrawler or earthworm is easier to digest then anything else, after not eating for so long it's better not to overload digestive system, but small dubia is fine too.

you said he is not eating, would you please describe how you were trying to feed him as well as what time a day, what food and where. how long has he been in his new smaller enclosure before you tried to feed him?

and you gotta try to lower humidity some more, you want 75-80, not 80-85.