Psychic plot theory
like everyone else I thought this movie was a mess. however it seems there is one clue as to the truth in the movie.
During the wasted scene with 30 seconds of Lucy Lawless the father says that the kid's imagination is really vivid and that "you know what will happen, if he think's it's real, it will be" or something like that.
So the boogeyman is actually a physical creation of Tim's mind. He has a psychic ability. This explains why he is visited by his mother. She knows his ability is what killed his father and he can see her at the moment of death because of this ability. Tim imagined the creature into existence after hearing the story and it killed his father. This also explains why he can see the girl.
The things in his room provide fuel for his imagination for the things in the movie ... when he starts thinking about his childhood house a bird flies in his car ... just like the model bird in his room.
The boogeyman is weakened when he smashes the model figure because the figure is what his mind used to give the monster a form.
As for the portals ... best I can do there is that in Tim's mind his creation can travel through the darkness of closets, doors etc. from place to place. As a result so can he. The portals become real. And because Tim manifests things with his mind, when he wants to see what happened to the victims, he imagines that he will come out at that time and he does.
So in summary. The creature is real. Tim created the creature with his mind when he was kid and let it loose on the town. Since it was created by his mind only he could destroy it, by destroying all the things from his childhood that he used to summon it in the first place.