Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
Oh, this poor little guy. Sounds like you are doing a great nursing care job. The absolute best for this little puppy is to take him to be seen on emergency at your veterinarian, or ER clinic. He is too little to tolerate not eating for very long. Our young puppies are very prone to dehydration.
As far as the cause, I am afraid I could not guess without performing an exam. Sometimes there is something congenital he might have been born with that may be causing failure. Sometimes this issue, if your vet identifies it can be treatable, and sometimes not (if there is an organ problem). Sounds like you are orphan raising this puppy? IF so, then this does add in a higher risk for pneumonia from aspiration bottle fed food and constipation. Those are potentially treatable issues but they can also be life-threatening at this age, and this is why an immediate exam is necessary.
Until you have him seen, continue to keep him warm (but not overheated) and offer him bottle food. if he is not an orphan but is not nursing from his mother, then you do need to bottle feed him, if he will take this. However, if he is too weak to even try, then this is yet another reason this is an emergency.
I wanted to add I see you added a request for a live phone call. Unfortunately, I cannot help with a phone call. The states/province I am licensed in do not allow me to communicate via phone calls, unless I have previously physically examined a pet. I could lose my license for doing so. If you would like I can opt out and see if there is another veterinarian that can make calls.