But in the moment, he forgets at all and appears to have no impulse control.
That sounds like he’s been triggered into fight or flight - which is a trauma response and something in the moment he doesn’t have control over. I agree with @Ted27 in that helping him not get to that point is probably your best move at this point, because he’s acting out of distress rather than temper or bad behaviour.
Lots of traumatised children really struggle with transitions so it makes sense that getting dressed, having his nappy changed would be triggers for him. Can you give him control over getting dressed eg what he wears, how long it takes etc? My two still can really find getting dressed difficult which means I get them up at 7 to leave for school at 8.45, we need the time so they can take as long as they need, if we’re staying around the house they may well stay in pjs all day.
My boy was 4 when placed and wasn’t fully toilet trained, we used pull ups because he could change them himself if he didn’t want my DH and I doing it. Is that an option? If he’s able to understand when he’s calm, explaining and giving him limited choices about what to wear, using a visual timetable so he can see when it’s time in his routine to get dressed, change his nappy etc might help him have a sense of control.
My DD can be very controlling too, depending on what the “not doing what he wants” is, I’d pick my battles. So playing with her, she’s in control, what she eats is within her control (we have a selection of snacks she can choose from, we all decide the weekly food menu together), she decides what she’s wearing on a given day etc. I save the “my way” for things that really need to be my way - and there are surprisingly few when it comes down to it.
The difficulty with a controlling child is that they can trigger the part of you that feels out of control so you end up in a bit of a power struggle, where you end up trying to regain control with a child who can’t tolerate that.
When he’s triggered to the point of violence it’s probably when he needs you most, he can’t regulate himself at that point, so needs you to help him with that. I need to work just now but I’ll pop back later if that’s ok.