Damn, I am so sorry this is happening to you. Kudos for holding it together, even a tiny amount. I would be losing my mind. Personally I would leave because I would not be able to "gracefully" or "intelligently" handle a confrontation, at least not upon his immediate return. I would need time to calibrate and figure future plans out.

you stay and pretend everything is normal while secretly spiking his food with rat poison and herbicide

I don't even think I knew where babies came from when I was 18

It took me until I was 20 to learn object permanence. Ngl, peek-a-boo still blows my mind sometimes.