My Daughter Boyfriend (28 Years Old) Has A High Fever

My daughter boyfriend (28 years old) has a high fever (up to 39) for 4 day already, it dropped down few times but then rise again. He also has a headache and neck pain. When temperature drops, his headache getting less intense . What possible it can be ?( He is not sensitive to light and not vomiting - I was trying to google his symptoms and miningites cam as a first suggestions :))
What possible it can be ?
Thank you