Hi welcome to Healthcare Magic.
Nght mares are common during 4-8 years age kids, they are a part of normal child development
, so you need not worry about them.
Regarding what you can do to help him , just before he goes to bed do not tell him any scary stories or scary incidents , dont allow him to watch scary movies on television. The rationale is that usually they dream about things they thought while just going to bed.
So donot worry this will pass by time ...
Hope i was able to relieve your anxiety