How to help son with nightmares?

My son has been having a lot of nightmares lately. He will wake up covered in sweat. Once he wakes up from a nightmare he won't go back to sleep for the night. He used to just come sleep in the bed with me if he couldn't get back to sleep, but it makes my boyfriend uncomfortable because he's in his teens. When he talks about his nightmares they are very disturbing. He's been talking to this therapist about them but he's still having them. Since he doesn't go back to sleep after he wakes up he's been falling asleep at school. Does anyone have any advice for nightmares in teens ?