How do I act?

[MrBond]

Your H is acting like a typical 25 y.o. male.

The biggest threat to them is authority. You are the mature one right now. Stand strong. He's going to be throwing his tantrums soon.

He's like the kid in the playground that likes kids to play by his rules. Then if you don't, he says "well I'm not going to be your friend". So call his bluff and don't play his game. This is exactly what's going to happen, he'll hold his breath, stomp around to get your attention, have you "overhear" things, etc. But then after all that is worn out and figures out that you're not playing his games, he'll have no one else to look at to blame but himself.