The big thing with horror is atmosphere. Ghostly noises in the dark, eerie moonlight, that kind of stuff. Let them expect bad things behind every corner, but actually have bad things there only rarely.
Always describe monsters lively and without stating what they are. Let them encounter something truely nasty sometimes, let them escape barely and then use this moment to play up their fears by reusing certain elements of such an encounter (i.e. the encompanying darkness or winds, just a few similarities to remind them). Throw in another survivor once, starting the encounter in a way, that's also similar to a monster encounter, so they will have to realize that they not only have to be on their toes, but also need to be careful to distinguish between friend and foe (that's especially good if that boy is turning against them at some point... confusing them even further whom to trust).
Bye
Thanee