Hello, I Was Wondering What My Brother Might Be Sick

Hello, I was wondering what my brother might be sick with.... he is eight years old and he was fine all day, but he went to soccer practice around 5 pm and he had a headache and a fever of 101 degrees. When he got home, we took his temp and he went straight to bed without eating or changing out of his soccer uniform. Do you have an idea of what he could have? Thank you so much.