Looks like he hasn't made connection with food and water yet. Failure to thrive is about to finalize itself.
If in the US I would suggest admisintering
Poultry nutri drench direct to the beak just below the nostrils, using the bend in your forefinger to help direct the drop and neak together. The PND is patented for rapid absorption and does not require digestion to work.
The red leg is something I've never seen before.. Is there possibly something wrapped around it cutting off circulation a little? Possibly something internal.. I'm guesssing you haven't seen it poo yet?
Was this chick hatched at home from your own flock, or shipped, etc? Hatch on day 21?
My favorite technical hatching resource has possible causes of specific things seen starting around page 51 in the following link.. slightly before that it discusses chick quality and redness of hocks is mentioned (p. 49).. maybe this is a more extreem version of the redenned hock??
https://www.hubbardbreeders.com/media/incubation_guideen__053407700_1525_26062017.pdf
Is there MORE heat coming from the red leg than the other one?
Hoping for a miracle for you both!