IIRC just keep the yolk sack moist and super clean and your hatchling should absorb it on their own - I'm not clear on the details of how moist or how long the absorbing part takes though, I just know you're not supposed to let the sack dry out.
Hopefully one of the breeders will be along to give a more complete answer :-/
I had one like that and it was so weak it just laid there for days without movement only if you "poked" at it. I don't know how your is doing but after several days I decided to put him to sleep
I would try and feed him something and hope for the best.
I had one that did not absorb its yolk... it survived just fine, and is now thriving at my cousins house. I just kept her on paper towel with a moist hide, and kept her nice and warm. She did what she had to do, and in a day she was back to "normal".
This is not that uncommon. Usually the hatchlings do just fine, even thought they are all 'caved in' looking. After they shed and start eating, they catch up in a short time.
We have encountered this problem also. She is fine now. we just wet a paper towel, put it in a small sepetate container, put the hatchling in it, then popped it back in the incubator. Warm and damp until first shed seemed to help.