Hi and welcome to YT.
First, this little baby shouldn't have been taken from his Momma until 12 weeks old - so that's part of why you're seeing such vulnerability here - coupled w/ his size etcetera.
You need to go back to whatever the BREEDER was feeding him, and stick to only that for a good 3 weeks before making *any* changes at all. He's only been alive for 8 weeks....remember that - he is very vulnerable from being taken from his Mom and littermates, causing great stress on its own. During this time of big change/stress...you do not want to make any more changes for this little one.
Also give the supplement that the vet told you to. And you've got to figure out a way to keep this kiddo warm. Get a hot water bottle perhaps. They also make heated beds, but they're not cheap.
As you can now see, when something goes wrong w/ these babies - it can go wrong VERY fast - so you did the absolute right thing in going immediately to the vet.
Keep us posted on how this little one does, hang in there, and just know that with proper care you'll be able to take care of him
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