After looking at the pictures carefully, he DOES look skinny. That belly is very sunken in. I'm not sure if that hatchling ever ate ?? It shouldn't have been sold like that, that's for sure, but what's done is done.
It is a good thing you are keeping it in a tub. You can get a smaller waterbowl and make room for 2 hides. One at each end, cool and warm end. Far more important then a huge water bowl. Hides should be dark and not to big.
Her is the big and most important question of them all. Do you have a thermostat on the heat tape ? If not, that is one thing that is absolutely critical. You need to control the heat output. Ditch the heat bulb. It just dries out the enclosure and it may just make it to hot. Your top should be solid to make the hatchling feel more secure. If you have the hide over the warm end, it will be cozy in there. you can wrap a towel around the tub to make it feel more secure and not to let cold in or heat out. Just leave one side open and a bit of the lid, so you can do a quick visual check.
Right now you have to have your husbandry 100% spot on. That includes heat, humidity 2 hides and privacy. You don't have much time to get everything right, that hatchling needs to eat before to long.
But your best bet to get it to eat is to have everything just right, and then leave it be for at least 5 days. No handling, no messing about other then quick visual checks and perhaps a light misting to keep the humidity going.
After the 5 or so days, offer a live mouse hopper or a rat fuzzy. Close the tub and watch from some distance, you want the hatchling to feel safe. They will not eat if they don't feel safe.
Hopefully that will work.
However, there is a chance it just won't eat. I doubt it ate for the breeder. Btw. What does the breeder say ??? About the weight discrepancy ? Whether it ate and how much ??
If NOTHING works, is there someone knowledgeable near you that could teach you how to assist feed ? This one might just need a jump start. But do not attempt to do this on your own, you have to try everything else first. Because the stress of that can make things a lot worse.
Good luck, that sure is a little BEAUTY !!!