Hi,
Fever and abnormal posturing of head and neck could mean that he has a meningitis. That would be top of my list here. I have seen a few recently and they generally respond very well to steroids (steroid responsive meningitis) but ideally should have a CSF tap to look at the spinal fluid for bacterial involvement. These cases should have prolonged antibiotic cover.
Notes from the web: Puppies with steroid responsive meningitis
usually have a fever, they have a stiff or uncoordinated gait, they often
resist having their head and neck touched or manipulated and may show signs of
severe pain. Paralysis, severe weakness or seizures may also occur.
There are other differential diagnoses but this is the top of my list. Others include middle ear disease and puppy strangles
hope that helps
David