What could be causing a 3 week headache and now nausea?

My husband has had a headache off and on for about 3 weeks. He has been to the doctor twice; both times it was diagnosed as a tension headache. The headache had been bothering him on the sides of his head, but now it's mainly in his forehead and under his eyes. His vision is fine (and he had it checked a couple of weeks ago), but he says he feels almost congested under his eyes and fullness in his forehead. The headache is never severe and he notices it more when he's not busy. Over the last week or so, he has been complaining of nausea; not enough to stop him from working, but it's still bothering him. He told me today that he's afraid it could be a brain tumour. I know he is very anxious about these things and although I am making him go to the doctor again (hopefully tonight or tomorrow), I want to be able to reure him. Other than the pressure/headache (which doesn't usually bother him until later in the day) and occasional nausea, he has no other symptoms, except for that his cheeks feel warm and look a bit flushed in the evening. He had a head cold/flu about a month ago, but doesn't have any obvious nasal congestion or discharge now.