Cigwyllt is right. If the dog is on IV and the vet doesn't know what is wrong, then nobody here can guess.
When you say the vet doesn't know, do you mean that the results from the test are not back yet, or that they just don't know?

Also, if the pup is only 6 weeks old, that is far too early for it to be taken from it's mom.
Hope all works out.