Children don't develop meaningful self control and decision making until age 3-5. He's still developing the capacity to make decisions about not doing something he wants to. I would argue that the moral/explaining/understanding part of the equation is never even invoked because the capability to invoke it hasn't physically developed yet.
It's never a binary thing, though. You can't just one day wake up and say 'son, now you're old enough to know better'. If you do so, a smart kid will simply ask 'so dad, what changed since yesterday?'.
The point is that nothing did change. Children don't go from animals to sentient beings overnight - they develop gradually.
So how do you deal with that? You take them seriously, and you accept that it isn't going to work at first, and then will work imperfectly, but will work fully when they're ready for it.
Yes, something did change, or else why are you changing the rules? Your understanding of them changes. Your opinion changes. Something had to change. And even if you don't know when it changed, you can still explain that it has.
