My husband has constant headache, what could it be?

See a doctor. Any ha that is constant deserves a full evaluation. Imaging may or may not be needed. Only a face to face visit here really works due to the need for a thorough interview and exam. I am all for trying to accomplish health care without office visits but that only applies some of the time. This needs to be looked into by his pcp for starters. A neurology second opinion might be helpful.