'My Son' is an ok film for a one-time watch. It looks neat, McAvoy is consistently good, there are some tense scenes (although very dark), and the score is memorable.
Yet it's still 5/10 because the story is just too loose. More than half of the things they establish go nowhere while the resolution just turns up from nowhere. So many plot threads just get abandoned constantly taking you out of the film, there's nothing to grasp on to. It's ok to mislead your viewers if you do it in a clever way, provide hints, well, at least something. Here it's just random. Some of the scenes also felt stretched out to meet the required run-time, I have no other explanation, lots of walking, running, sitting, driving...
The ending credits also mention that this is an adaptation of Mon garçon, a French film of the same name from 2017. But how can it be an adaptation if it's directed by the same guy? It's like he just made the same film twice, and the first one isn't much better probably because it's rated even lower.
It's watchable, but only if there is nothing else to see.