I saw this in theaters shortly after it came out, and while I didn't love it, I thought it was sort of interesting. Seeing it again for the first time in a few years, I pretty much feel the same way, which, in this case, is mostly positive.
The best part about this film is the atmosphere, hands down. It's a dim mansion, and while there are jump scares, I feel more of the frightening portions are subtle. There's a dream sequence I could have done without, but for the most part, I think the scares come honestly.
What helps is the cast of about two people. Sure, Brahms' parents, Jim Norton and Diana Hardcastle are both fantastic, but neither has much screen-time. Ben Robson doesn't show up until the end, and he's not exactly oozing with interesting character traits (though to be fair, it's more due to the script than Robson himself).
So who we have to entertain us for most of the film, providing you don't count the creepy Brahms doll, are Lauren Cohan and Rupert Evans. Cohan does a pretty fair job throughout, and after discovering the doll's more active than it should be, really comes across as crazy at times. Evans, who has previously appeared in such horror films as Asylum Blackout (or The Incident) from 2011 and The Canal from 2014, has a very enjoyable performance here, and really, he comes across as quite charming. Luckily, Cohan and Evans worked quite well together, which is great, as they were about the only important characters in the film.
Gore certainly wasn't much a factor here, and really, special effects weren't needed, as the film keeps things pretty simple. Like I said, it's the atmosphere that's most commendable, but certainly the performances listed above help out. What also can't be ignored is the moderately creepy mansion, which looked great and certainly helped add to the already well-done atmosphere.
The biggest issue with The Boy is that it sort of meanders a bit toward the middle of the film, which was fine for character building, but it's not the most exciting material. That said, I did like how Cohan's character's personal issues tied in well enough to the film's plot, and certainly gave her reason to want to stay after discovering the doll she was hired to watch over was alive.
The Boy didn't really garner much attention when it came out (the director, William Brent Bell, hasn't done that much before this, though he did director 2013's Wer, one of the more interesting modern werewolf films), and I can sort of see why. Personally, I think it's an enjoyable film with some solid acting, good suspense and atmosphere, and delightful misdirection, leading to a rather fun conclusion. While not a masterpiece, upon seeing this film again, and outside of a theater experience, I can say that it's a solid film and I'd recommend it.