INFPs are quite sensitive to external negative criticism - and if he's been neglected/mildly abused, I think it would have a deeper impact on an INFP than some other types.Are INFPs more susceptible to the effects of certain parental neglect?
You seem a little resistant against typing him INFP, though, despite his arguments - why is that? Anything you're spotting in him that might not come through so clearly here?
You said you see Si and Fe in him.... INFPs are Fi Ne Si Te, so you could quite easily be seeing his tertiary function coming out.
So then that leaves Fe.
A combination of Ne and Fi (in ENFPs) often give off the impression of Fe, although it is more momentary than true Fe. I'm not sure if the INFP gives off a similar impression (seeing as they're using the same two functions), though.
Fi, with it's strong internal compass, is often driven to help people, if that's what he values - are these strangers that he gives dangerously to obviously disadvantaged, or can it be just anyone?
Yeah, I don't know if I'm going in circles or over what you've already thought of, but hopefully that might help a little?