Hey, just need some assistance with my Savannah Monitor.?

Well I bought "Swisher" 3 days ago from Craigslist(savannah monitor)- he is about 6-7" long- the guy said he was unsure of the age and sex of the monitor, so your guess is as good as mine..?..for right now he is in a 55 gal tank with some kind of red wood chips from petsmart, a ceramic rock heater,water dish(he took one dip in it) and a little brick house he can hide in. Hi side stays 85-105.. With humidity from 30-50...Swisher was not tame and was the reason the guy didn't want him, however, when I set up his enclosure he was walking around and exploring-later in the day he ate about 4 baby hoppers...the day after he hasn't really been moving- he varied from about 3 spots and lays under the rock with his eyes closed- I just want to make sure he is not dehydrated and what a monitor's behavior this size should be at? This is my first monitor, I am a snake guy but I like new and I want the best for my little guy. Thank you