Is his way of hanging on just a fear of rejection? Or is it a deep attachment?
Well, I had a really long response, but my computer deleted it so I'll be a little less verbose.
INTJs are close to very few people. We don't make friends easily and are usually slow to open up to them and become close. Being open with others is... risky (best word I can think of) in our minds. We open up to somebody, become close, and put ourselves in a position where they can hurt us, not through the usual means (though we fear those too) but by rejection of ourselves.
I mean this rejection in a different way than usual. With most people, INTJs are somewhat aloof and self-confident. Because all other people see is the outer shell, our masks and the front or normality we put on for the rest of the world. When they reject us, we don't care. They're only rejecting our mask. It is only to a select few that we take off our mask, our protection for.
By showing somebody, carefully chosen and trusted, our inner, honest selves, we become very sensitive to rejection from that person because we can't tell where it's coming from.
Your INTJ probably doesn't understand that you're rejecting him because of differing principles (we're not very good with interpersonal relationships as a rule), he just sees that you're rejecting him and is assuming that you're rejecting
him. His whole honest self that he has fearfully and hesitatingly revealed to you.
To answer your original question, his 'hanging on' is both an expression of a fear of rejection AND a sign of deep attachment.
Because he wouldn't fear your rejection if he wasn't deeply attached to you. It is because he is very close to you that he fears your rejection, because it would be unavoidable evidence (even to an INTJ) that he is unworthy (or a loser, or worthless; whatever it is he doesn't want to be, for me it's unworthy) to you.
He sees it this way: I open up and reveal myself to you. You reject me, I do not understand why. You reject my true self that I revealed to you. It hurts beyond all belief. I will try and hold on to you because I really like you and because your rejection will be the confirmation of all my worst fears (and undermine my self-confidence and convince me, once and for all, that relationships with people will only ever hurt me).
Not sure if I answered you well, but it makes (mostly) sense to me. If I wasn't clear, I'd be happy to try again.
(Edit: maybe I should make an INTJ help desk thread, where I answer questions the INTPs have about their J cousins. XP)