I think you have already - you've established those boundaries. If he still does not understand, maybe you could explain it again, making his realise that his indifference does hurt. I think you're right in that it's never ideal to dump a friend. Maybe you could explain in writing, like you've posted here, and just be sincere and objective like you are.
Apologies for the last-minute edit here, but something occurred to me too, just as I was getting yet another run-around message from the one who does it to me. Anyhow, I think at some point we just have to accept that certain individuals are just that way.....I know it hurts, I know it's not right (and we're not like that), but your friend has a flaw. You'll not be able to fix the flaw, and it's not your fault. Instead, just to accept that it's imperfect and hurts.
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The ones who say “You can’t” and “You won’t” are probably the ones scared that you will. - Unknown