Your husband has got Viral infection.It cannot be diagnosed with the above tests.You have to consult Virologist/Microbiologist for identification of the Correct virus.
Many viruses can cause high fever
and headache.But,the treatment is the same .
He has to go for full dose of Paracetamol
650 mg ,4 times a day.Add an antibiotic like Azithromycin
to prevent secondary infection.
Wish him speedy recovery