I didn’t read his reaction that way at all. He seemed pretty shocked. And all he said was he couldn’t leave Zelena in NYC, not that he was staying with her.
I went back to re-watch to pinpoint why it rubbed me the wrong way. It’s their choice of words, “I’m sorry, I can’t leave her here.” He’s just found out that the woman who killed his wife has been raping him to make a baby, and the way they worded it “I can’t leave her” just has the wrong connotation IMO. Had they had him say, “I’m sorry, we have to take her with us, because she’s carrying my child,” that would come across differently IMO. That would sound like he’s on Regina’s side, instead of almost sounding sympathetic towards Zelena.
I’m convinced this the pregnancy was all a ploy to get back in Storybrooke so she can get her real happy ending.
If so then it’s risky to fake it when it can so easily be dis-proven. If Regina’s got half a brain, she’ll make Zelena pee on a stick before she agrees to take her back to SB with them.