Looking at the points you made he seems very INFP. What makes me say that the most though is that I can relate to about all of those points VERY WELL.
I would say that my mom is an ISFJ and her personality is much 'stronger' in a sense. She can be quite sensitive and emotional, but most of the time she doesn't let that get in the way of her responsibilities and actions (like I think it would an INFP--though they hold it in, it translates into dreaming and not action) until it all builds up (and blows up), meaning she's actually quite independent (and no codependent, as you asked) and though she does have my dad for help, she's able to step away from that if she needs to and say she can do it. There is still a lot of structure in her life despite problems (she didn't have the best childhood either, there was parental neglect).
INFPs can be very sensitive to parental neglect, even if there isn't really that much. We just tend to be sensitive, especially to any slight perception of rejection. I mean, of course we can grow out of that and rationalize that there really isn't anything there when we think there is, but if there is some kind of neglect going on, I think it hits INFPs even harder. We can be total wusses but hide it well.
The way he seems to 'drown in fantasy' and romantic notions, screams NF for idealist and having that strong desire for the perfect fulfilling relationships.
Anyway, I'm no good at figuring this out by function analysis, but anecdotally, I would be very sure to say he's probably an INFP. (He thinks he's one anyway. Why do you have to doubt him!?!!?)
I don't even really see how these are more ISFJ than INFP
--He used to look at art constantly, forming a loose library of his favourites, as inspiration, on his computer
--Related to that, he's done very skilled art himself
--He's always loved to help people, and he'll often dangerously give himself and his attention, even to strangers
His Magic Tree House thing seems really not a thing an ISFJ would draw... and I find it really weird how he has a giant squid monster in his gallery because that's something I love to draw... And I think that though both INFPs and ISFJs like to help people, INFPs tend to do it more randomly, to whatever cause they find around them, while ISFJs tend to go into some kind of social program and dedicate themselves to that or they help a few people very close to them to a high degree. I think the type of help they tend to provide would have differences too. INFPs are good at giving moral support, attention, acceptance and good feelings, and probably loose change. ISFJs give a lot of their emotions as well but are also really practical and give good advice and maybe more likely financial support. I don't know for sure, that's just observational speculation... but though INFPs and ISFJs share a lot of traits there are very different ways in which they act out.
Anyway, to reiterate. INFP INFP. Maybe a relatively unhealthy one though, so he may exhibit traits contrary to his type that SJs may have.