I found the origami killer and saved Shaun, but in the proccess
I had Ethan drink the poison (thus he must have died, right?).But in a cutscene right at the end, it showed Madison, Ethan and Shaun buying a new house together and being happy. Glitch, dream, poison not really lethal, etc...?

It was not really Poison, just another trick from the Origami Killer to test Ethan to see how far he would go in saving his son.