"Ditto to Steffke's recommendations on lighting and temperature. Keep an eye on humidity also. Daily soaks and humid substrate are tremendously important. A baby box turtle will lose its appetite very quickly if it becomes dehydrated.
"If I feed my baby turtles mainly earthworms (sometimes, they're all I have on hand), I have noticed that they don't put on weight or grow very much. I think that higher fat items need to be a regular part of their diet, when they're very young, to keep them pleasantly plump and growing. I decrease the frequency of high fat food items as they get larger. I only mention this because you stated that the turtle was eating one worm per day. If it were me, I'd feed dusted waxworms at least once per week. I hope this helps. Good luck with the little one."
I USED to feed "him" one earthworm per day when the little one was very young. "He" won't take 'em anymore. The most common food item now is wax worms, which I dust with calcium.
A/C is on in my home on the rather warm days, but the wall A/C unit is quite some distance from the box turtle tank. Temperaures where he is are higher now than they are in winter. I did stop spraying the Sphagnum, but no from what you say, I probably ought to. Seemed like he needed moisture less, since when I happened to let the Sphagnum dry out, I figured if dehydrated, he would spend more time in the soakk - but even with dry Sphagnum, soaks are far shorter these days than when I first got "him".
You baby three-toed looks cuter than mine.