M uch as he surely hates the word, Québecois hipster hero Xavier Dolan is a filmmaker for whom the noun wunderkind might have been invented. He now has five films under his belt at a revolting 25 years old, the same age when Orson Welles just had the one. Albeit a fairly good one.

Mommy, Dolan’s latest, is his funniest. It comes at you baying and rattling like an early Pedro Almodóvar comedy, threaded through with an infectious love of full-throttle melodrama, and flinging its energy right back to the cheap seats, thanks to Dolan’s customarily zippy design choices.

The film has three characters on different verges of entirely different nervous breakdowns – a mother and her 15-year-old son, played by Anne Dorval and Antoine-Olivier Pilon, and their neighbour across the street (Suzanne Clément), an anxious little bird of a teacher with a speech impediment.

This threesome dominate the film so wholly it takes the shape of a chamber trio, in a near-constant state of presto agitato. Pilon’s Steve, an unstable rocket of adolescent energy with severe ADHD, has just been discharged from a juvenile care facility following his latest stunt, which left a peer with life-altering facial burns. Dorval’s Diane (“Die”) is his widowed single mother, who hangs out with him more like a sister or on-off girlfriend than your average suburban mom in Montreal. She typically looks dressed for a trampy night out knocking back flaming sambucas, and that’s just when she’s making breakfast.

These two have a more or less co-dependent relationship, but a stabilising influence is clearly needed, and this is where Clément’s Kyla comes in. She’s drawn into their crazy orbit, agreeing to help Steve with free tuition to provide the bedrock for leaving his delinquent days behind.

Something about Die and Steve’s mad devotion to each other, and soon enough to their new ally as well, quells Kyla’s demons – she crosses the street to see them, and the conversation flows, but she reverts to her dithering, neurotic self when back with her own clan.

Rupert Grint on speed: Antoine-Olivier Pilon in ’;Mommy’; Credit: Shayne Laverdiere

“Maybe/ You’re gonna be the one who saves me. ” blares out Liam Gallagher in the chorus of Wonderwall, which Dolan co-opts for a sunny, it’s-all-working-out montage at the halfway point. Maybe. Maybe not. If Dolan almost gets away with this supremely on-the-nose scoring choice – typical of a so-uncool-it’s-cool soundtrack of 1990s hits from Counting Crows and even Dido – it’s because the sequence in question is an undeniably giddy sugar rush building to an ecstatic formal coup. Most of the film is shot in the almost unheard-of 1:1 aspect ratio – a square frame rather too obviously hemming in this trio’s horizons. But Steve, skateboarding down their street, flings his arms wide after a full hour of screen time, and pushes the frame open like a magician.

It would serve the film better to stay wide, ring the changes, and maybe modulate its tone at this point. Dolan’s female stars deliver high-intensity characterisations with a touch of Mike Leigh in their DNA. They’re never boring, but there’s no middle to either of these parts, and both actresses were even better in a couple of Dolan’s previous films – Clément in the transsexual romance Laurence Anyways (2012); Dorval in his debut, I Killed My Mother (2009).

The truly fresh ingredient here is Pilon. If you can imagine Rupert Grint gone peroxide-blond, doing obscene things with his face every few seconds, and rampaging through his scenes as if every day were his last on the planet, this is the star-making turn we get. It’s the poutiest male performance since Zoolander. As a character, Steve is wildly obnoxious, unforgivably racist, almost film-ruiningly aggravating, and you can’t take your eyes off him.

Dolan has everything in his arsenal now except discipline, which he appears to have left under the bed after the most promising film of his Icarus-like career, last year’s brilliantly atonal Tom at the Farm. The final 20 minutes of Mommy are the least convincing, despite another ingenious, crowd-pleasing montage Dolan has up his sleeve. There’s something a little vain about any film that keeps trying to insist its characters are Too Full Of Life for a conventional happy ending. Vain? Dolan? Always. Bouncing off its own square walls, his movie has all the reckless faults its people do, which you suspect is exactly how he wants it.