@footprintsintheslow were in a similar ish position to you, DD is 16w and DS will be 4 in June.
I’ve done the same as you, plenty of 1-2-1 attention etc, reinforcing how loved and important he is, reinforcing the positive behaviours ‘DS I saw how wonderfully you can colour’ ‘DS when I asked you to do XYZ you did such a good job and I’m proud of you’ about 20 times a day, just the really mundane things, ‘what good manners you have using the coaster, DD will have to do that when she’s grown up just like you’
Tbf at first he wasn’t bothered about her, then once the reality of me stuck BFing her 20 million hours a day hit home his behaviour escalated and the TV was on too much, which exacerbates it.
So the telly is off much more. I spend 20 mins most nights getting a couple of little activities set up and left out or ready to be popped out when he’s not looking/distracted, I’ll get a toy out and start playing with it until he joins in. Essentially I’m love bombing him multiple times a day for a few minutes.
He responds well to affirmations of family, so we do family cuddles, we take it in turns to get in the middle, like a sandwich and about ‘mummy sandwich!’ Then swap the family member in the middle until we’ve all been the sandwich filling. We shout out things like ‘who’s my favourite boy?’ And he shouts back that it’s him.
And when it all goes to shit I make DP take him out for a run to the park or whatever just to totally reset.