A few suggestions:
- Monster's Inc (absolutely great)
- Some episodes of Dreamwork's trollhunters animated series
(but they might be too scary depending on the age of your boy. Also, not about demons but about trolls)
- Possibly "How to train your dragon" movies & episodes (again, depending on the age of your boy. Also, not demons but dragons).
There's also a beautiful children's book by Ottfried Preussler called "The Little Ghost", which I bet they must have made into a movie. But I'd suggest you read it to him as a bedtime story.
With my kids, it also helped to humor them. If they were afraid of monsters in their room, we went together to check for them under the bed and behind the door. Plus their stuffed animals had awesome monster-fighting powers.