You can see the moment Isak has connected the dots in his head after Even’s weird wedding speech. He must know the signs from his mother, but he seems to want to ignore it or hope its not really happening. But he knows.
Then, when Even disappears out and he goes to check the corridor, his face tells you everything. He knows.
And finally when Sonja confirms it, his face isn’t shocked or surprised. You can see the truth set in his eyes. He knew.