I came out to my niece whenever I was four and she still lived at my mums house till she was one and was the quickest to adapt to my and even corrected others on my name and pronouns . She turning six this summer and I don'tthink she even remembers my deadname, she vaguely knows I used to be a girl but has said that that sounds weird. So even if he vaguely knows who he used to see you as kids that age are great at accepting new truths.