I think that ultimately it’s Henry’s heart that PP is after, and he’s testing its strength by putting Henry in a situation where he’s in danger, and has to keep the faith against mounting odds that his family will save him.
But I also think he’s testing Emma for some particular purpose as well. This is PP we’re talking about, so mummy issues and testing the familial bond to see if it truly exists is something that would totally fit. And along those same lines, I suspect we might see him testing Regina as well, (her promo poster did focus on a mother’s love being able to overcome). Perhaps him testing Emma is intended to have a flow on effect of testing Regina at the same time, seeing if the two mothers who are typically very antagonistic, can actually come together and play nice for the sake of saving their son.