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James is very close to his grandfather who he talks about being his mentor and always being there for him. He seems to have an affinity with older people a great deal of respect for them. I think in part he was genuine in picking Janice for Chloe's sake. That clip in the garden last night where Bobby was talking to Chloe about her reliance on Janice made me realize that James wasn't exaggerating and she really did need Janice around although we don't get to see that really.
I think it was also partly because of the way Austin had spoken to Janice earlier that day. Maybe he felt sorry for her or maybe it was a 'take that!' to Austin. Or maybe he wanted to be the hero. I think a combination of everything seems reasonable. There is rarely just one thought process that goes into any decision, more a mixture of reasons and emotions...