before he left for China if she want marriage so bad. They were still together in Oct, then the kid was born next year May. So it mean before he left for China trip, the mom already knew and may talk about it. He must heard about it and didn't want to commit to marriage. Then he said no to marriage and left for China for 2 months.
That is why he was sure the kid is his even before DNA test.
The past is past, but the only thing is he has only his feeling in his heart, no other