OP is original posterSorry I misunderstood. When you said you were taking him back to the pet shop, I thought you were returning him. Is there any way you can get him set up with an under tank heater and a thermostat? It's over 90 in my house, and everyone still has their heat and t-stats on. When it gets too hot, they simply shut off. It'll be much easier for you in the long run, as you won't have to constantly monitor and worry about his set up.
You'd be surprised how big of a feeder the little guys can take down. It's probably not too big for him, but if you're concerned I'd definitely get a live rat pinky if you can get your hands on one. But offer it after he's been settled for awhile and isn't stressed.