Hi Mooshie,
I suffered really bad nightmares when I was little... I did for years... they still get me every now and again now.
Mine would come hard and fast when I was anxious about something.. it could have been a little thing or a huge thing.
Maybe just try and pry out of him what is going on at school... is everthing ok at home... I grew up in a pretty unstable home life so that was the core cause of my bad dreams.. Since i have been happily married and had DS they are now just a nightly show of bits and pieces.

Also when you put him to bed... give him a BIG hug and a kiss on the forehead.... it will help earth him out, think lots of love and thoughts of you protecting him while you hug him very powerful the old hug.... specially one from your mum!!
Whe I would wake up from my nightmares in the middle of the night my mum would just hug me to her heart and I would fall back to sleep in no time When she couldnt do that for reasons best forgot... they would keep going through the night....

Sorry bit of a ramble there!! But try the hug to the heart before bed

Good luck
xoxoxoxooxoxox