13 day old kitten "boney" but not "skinny"

Tomorrow marks the two week mark of me having my litter of 6 foster kittens.

Everyone was doing fine, and even now,5 of them are fat little balls of cute, they eat well, they're growing, eyes opening ect.

then there is one, who...is quite off. 

this little guy is confusing, his face isn't round, like his siblings, it's very narrow, almost siamese looking. He's smaller than his brothers and sisters, and he's very...boney...but he's not skinny, I can feel his spine, his ribs and his legs and toe/finger bones are very boney looking.

but he's got a belly, he spends as much time on the bottle as the others, but is still very lathargic, his nose and paw pads are an offwhite/pink color. But he's cold to the touch.

I've still got a huge problem with fleas, and this morning finally broke down and bathed the kittens, but I'm not sure why it's just this one. 

I took him to the vet the other day and they said it was failure to thrive and suggested I put him down.

But I didn't want to do that, so I brought him home and now I'm looking for a second opinion.

As far as development, he's right on track, his eyes are opening just like his siblings.

trying to get a picture to upload now.

Do you weigh them?  Seeing the weight changes - hopefully additions of 10+ g every day - is a good help for most.  Here not least, to know if he is "just" boney because of a genetic cause, depending if he perhaps got a Oriental gene, OR dont thriving.

You can try and carry him on you, at the chest, as often you can. Effectful both for weak human babies and kittens.

Make perhaps a little body-cloth of some thin sock?

Him being thinner than the others contributes too he isnt so warm.  But he must be warm to digest properly...  So fight on with that.

Good luck!