Jack Russel Puppy bloated chest / stomach

Question
I got my first JR and have no idea what size he should be for a six week old pup. The vet told me he's the runt of the litter. He's very small with a bloated diafragma / top stomach. The sections of his body just above his hips are small, it's the top section that's bloated. The vet wanted to put him out as his heartbeat was 50 pm and his temerature was very low but after warming him up and giving him antibiotics, he's much better.
Does anyone know what this bloatedness means?

Answer
Hi Heidi,
First of all, you shouldn't have a SIX week old puppy. He is at least TWO WEEKS too young to be away from his mother. This infuriates me because it seems like whoever you got him from is much more interested in the money they received from you than they do about the health of the puppy.
His little digestive system is still quite premature and he should be on a milk supplement such as Esbilac and crushed food for at least the next two weeks.
Warmth at this age is also crucial.
You said you took him to the vet which is good. Did the vet test him for PARVOVIRUS which is a dangerous virus that can kill a puppy in a matter of hours. What did the vet say about his stomach? Why didn't you get an answer?  It's hard to say exactly what it is if I can't see or examine him.  He also needs his first set of vaccinations at about 7 weeks, which a good breeder usually does for you, but it's unlikely in this case.
If the distention in his abdomen doesn't go away, please bring him back to the veterinarian for further evaluation. Keep him warm and go easy with the puppy food making sure to make it easily digestible by adding the Esbilac and crushing up dry food. Don't use canned wet food, it's terrible for them.  
Good luck with your new little guy, you are kind and good by reaching out and asking for help... he didn't get the very best start by being plucked away so early from his mom and siblings, but there is hope because of you.