Is there any therapy to stop my 8yo son from having nightmare most of the time?

Avoid fatigue: Nightmares are more common when children are overtired. Make sure you have an early bedtime for your child and stick to the schedule whenever possible. Have a nice bedtime routine as well (bath, brushing teeth, stories, lullabies). If your child seems stressed or anxious going to bed talk with them and try to figure why they are feeling that way. If the dark is scary consider a night light.

Quite likely: Need more information. Nightmares can come from either external sources or brain changes. If they are new and things in his environment have been changing (stress, trauma, bullying, etc.), the best approach may be to identify the source and help him come to terms with what happened. If there is a physiological cause, he may need a sleep study and to work with a sleep disorders expert.