NEED ADVICE ASAP: At what temperature should I incubate a duckling that hatched early with it's...

I have baby duck in the incubator on a damp hand towel and am trying to maintain the temperature at around 99 degrees Farenheit. She/he is scootching around some and is very weak but otherwise is doing pretty good all things considered.  

Is there anything else I should be doing?   

Thanks or your advice :) 

It is by itself.  All the others did alright.  It isn't dried out because I'm keeping it humid so the yolk sac doesn't tear.  It's too weak to hold it's head up much but the little guy is trying.  He/she isn't able to stand.  He's trying though.  But overall, just weak and shaky......he looks like he needed a couple more days in the egg.  And the yolk sac is/was a pretty good size.  I haven't handled him hardly at all to check how the sac is looking a I don't want to minimize risk of infection.  I'm kind of doubtful that he/she is able to drink or eat at this stage. 

I have mama and sibs in the brooder as there isn't a safe place for them in the coop or elsewhere.  He's just too weak for being in a brooder altogether. 

I'll try some water w/ Nutri-drench and see if he'll take it.  Should I dry him off?  If he needs to stay in the incubator, what temperature do you reccommend?

Thank you for your help :)
 

So, I checked his tummy for the yolk sac and to my amazement it's all but gone!  He's peeeping and trying to get about but is very wobbly.  Can't get him to drink yet.  He is sitting up albeit very wobbly.   I'd say he's definitely not ready for the brooder yet.

What do you think would be the next step?

Thanks again :)