Why did this trigger Dutch so much?

I recently just finished the game for the second time and this scene had me thinking. At first I thought it was obvious that it was because Arthur was no longer just following his lead, but he stop doing that way before this interaction. He’s been doubting and doing things behind Dutch back leading up to this but this moment is where it seems like he snapped. He doesn’t even call him son after but instead he calls him “pal”.