
It sounds like he is fading, to only be 3 ounces at 3 weeks of intensive feeding regime makes me think he has some serious health problems, he should wee and poo every feed and should have a strong suck to stop milk coming down his nose[I presume his palates are intact], it could equally go down into his lungs and cause pneumonia.
You shouldn't feed a pup who is cold as he will be lethargic and all his systems including digestion will have slowed right down causing more problems. Did the vet get him to poo? Usually a well lubricated thermometer has the desired effect provided there is no obstruction.
I am sorry to say things don't look too hopeful for this little chap and if he doesn't start thriving it may be kinder to have the vet put him to sleep.