Well we have some success over the past few days since posting.

Some thoughts first.

When I was telling him he was losing screen time, the thought was flashing through my mind that he will forget and it won't be a meaningful consequence but I have continued with it, he hasn't had any TV or iPad since then, for two reasons.

1, I said that would be the case and I intend to stick to it, even though I know he doesn't quite get it, because...
2, I feel like some screen-free time could be only good.

Sure enough, that afternoon after DS2 had gone down, DS1 wanted to watch TV and I said no, let's play a board game. So I sat on the floor with him, just him, and played bingo and pictureka and trouble. Then we found this awesome Where's Wally book but with Santa and we spent a good hour poring over that looking for Santa and a whole lot of other characters

So I had come to the same conclusion as someone suggested above - bit more intentional one on one time.

He's seemed better today, calmer. Still a few outbursts but I think overall better.