How do I help my SO

Okay girls my boyfriend has been having this really bad nightmares that don't let him sleep it's usually the same dream and it sometimes happens once every week or so on he is currently in the army but his been having this nightmares way before. He was also a volunteer firefighter. When I ask him about he kinda changes the subject or says don't worry about it, but at nights he calls me all scared. Dose anyone have any advice?