What makes you doubt his fidelity? If I've learned one thing about INFJs-- the ones I've met anyway-- it's that they're faithful and loyal to a fault and will stay in a relationship long after most other types would have identified the situation as hopeless and moved on. They have an unassailable code of ethics.
Then again the INFJs I know were all women so take that for what it's worth.
yeah, pretty much pegs me. but.
i have this infj guy friend (who i keep in touch with via email) and he does love him some woman. haha. he's married in a very ltr, but they have some sort of 'understanding' between them; a don't-ask-don't-tell sort of arrangement......anyway, i like keeping in touch with him mainly (and i hate to admit this) because his stories of debauchery are so entertaining for me.
the sub-love-life is fascinating. that he really likes these girls, i've no doubt whatsoever. and it's not like he's a huge whore, imo. but there is always a side-line girl, for the most part. and if there's not, he's working it. but, anyway, he'll meet this young girl, usually 10-20 years younger than himself, and they'll share some mutual interest and have sex fairly quickly (like first night or sometimes a little later), then she'll be really into him, and bombard him with texts, pics, email, pcs, whatever.
from what i've seen, he's pretty
true to his new lover. so, he's not a playa in the full sense of the word. he isn't into her as much as she is him, probably because he has his real family and doesn't allow all his barriers to come down (in fact he tells me this--i'm like his only confidante in his deviant wanderings), but he does let them closer than he really intends. he currently has a tenacious one who is getting in further than he really (said he) wanted. but he's still looking on the sidelines, and has one or two or three others lined up and sorta waiting.........
mind you, i don't necessarily admire nor condone this. i just observe and marvel at human nature. i wonder about guys (people) to whom others flock and what makes them so flockable. i think he's such a chick magnet because he knows just what to say and says it with sincerity, but also because (as ladder theory already states...haha!) he's simply unavailable!
as to the OP: i think this goes beyond mbti for you maybe. you sound like you're in school. if it's high school, (or even college) you could simply be wary because he's a guy and you're a girl and guys want to get in girl's pants. young guys (all guys?) are (typically) horny and not always able to put your best interests above their own..............time will tell if he's worthy, infj or no.