
honestly....i wouldnt be worrying about doubling his birthweight as long as he IS feeding.you will (as others may tell you) sometimes get a smaller puppy,but ill bet in 3 weeks time hes rumaging around just as strong as his litter mates.
in my litter i had pups born at 450g and 600g.the 450 was the smallest but was soon the strong little pup and went on to be as healthy as the litter mates.
if it makes you feel better you can make sure that he feeds first,put him on the back nipplethen put three other pups on to feed to get mums milk to flow, when he's finished you can then allow the other pups to feed.
is it just him crying,theres no way the pups could be cold is there?i mean they could all be warm on a heat matt but a draft can cause pups to cry.i wish someone eles would advise you as im no expert iv just raised litters!
wish i could help more
edited to say i think as long as he gains weight and doesnt lose it then he should be fine. ;-)