That scene is heartbreaking to watch. Keoghan deserved the academy award for best supporting actor this year IMO (as for best screenplay, while I think Banshee's should've gotten it, it was pretty close and EEAAO would've been my second choice anyway),
Is it supposed to be a caricature of a mentally disabled person? What is great about that acting? Not being rhetorical here, I'm not a person of high culture.