David is understandably the focus of Laura’s instability, since he, an eight-year-old, was born right around when Laura escaped the above-mentioned cult. Also (and here’s where I’m obliged to throw in a NECESSARY SPOILER warning): David may or may not be a vampire. At least, that’s what his biological symptoms and generic behavior suggest, between gastrointestinal bleeding and festering skin rashes. David’s health only improves after he eats human flesh, so there’s that, too.

But there isn’t much of a mystery about what’s really going on in “Son.” Instead, so much of the movie is a pat is-she-or-isn’t-she crazy narrative about Laura, a single woman who may have made up a past trauma to explain sexual trauma that’s occasionally mentioned, but never seriously considered. Writer/director Ivan Kavanagh (“The Canal”) uses vampirism, or something similar, to reduce personal grief to a few trite emotional symbols. Unfortunately, the sins of David’s absent father and unreliable mother are barely addressed every time this kid either hemorrhages or drinks blood.

That bloodshed, as plentiful as it may be, is only so compelling given that most of “Son” is a doom-laden chase movie: on the run from the law, Laura must find a cure for her son before he (really, they) must kill again. The rest of the movie otherwise boils down to a horror trope bingo, including seedy motels, an homage to “The Shining” (remember the bathroom in room 237?), a violently horny pimp, a skittish (and very clammy) family member, and more.

There’s even a pair of temperamentally opposed cops, one who’s hard on Laura’s story, and another who’s kinda sweet on her. As nice cop Paul, Hirsch stands in for a lot of what’s left out of “Son,” namely a distinct personality. He squints and sighs with such a pained expression that one can’t help but wonder what his character’s deal is, and why it’s not in the movie.

Also missing from the movie: Laura’s cult, who are mostly absent beyond a few (genuinely creepy) glimpses of its plain-dressed members. That sort of crucial omission is presumably the movie’s point, since “Son” is about the gaps in Laura’s knowledge, and how they serve to protect her from the awful truth about her past. Still, these major elisions might be more compelling if, say, the cartoons that David watches post-feeding were more evocative. Or what if David’s demon father were represented beyond a few sentences? Maybe Paul’s research could tell him something more than an unbelievable “she made it all up,” to quote one psychiatrist? Something like that?

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