So is he totally without heat now, with that light turned off? That's not gonna work...
While a thermostat is BEST & SAFEST -until you get one with UTH hooked up- & IF all he has for heat now is that heat light...you should buy a "lamp dimmer" from your
nearest hardware or home improvement type of store; easiest to buy the kind already installed on an extension cord (& very handy for using on ANY lamps) with a slide-
control to dim the light so he's not being "cooked". Leaving him at ambient room temps. (you said 78*?) isn't going to keep him healthy or able to digest.
You also need to measure his temps. accurately. Obviously, it would have been best to set up his cage a week ahead of time & well before moving him into it,
so you could have made adjustments to temp. & humidity without stressing your new snake. (you'll know for next time?)