you are in deep shock, and grieving and spinning and hurt which is all entirely normal.
But if I could gently wonder, about him "opening up" re why he doesn't want the relationship to continue, how he owes you at least that? I wonder if there is any reason he could give that would make you feel better about this? I am willing to bet that almost any reason he gave would make you feel worse, either because it seems trivial, or is a further betrayal or is termed as something you have done/haven't done, or seemed to be a lie.
In truth, for his own reasons, however odd they may be, he does not want to continue the relationship, and when you do, that hurts like h*ll. Perhaps concentrate your energies, not on trying to find out why he feels this way ), but on you: building your strength, getting your support.
I'm not saying that to minimise your experience, just I know in the past I have pursued a "reason" and when I got it, I didn't experience closure, just more hurt (or disbelief), I could never change it, and often it just didn't make any objective sense, but the heart can be like that.
From now on, if someone doesn't want to be with me, that's all I need to know and the relationship is over for me too. I am not going to torture myself with what-ifs: I don't want to pursue anything with someone who doesn't want to be with me (feelings of course take a while to catch up.....)
re the friend he went to for advice? he went to where he knew he would get support for the decisions he had already made, we all do that to a certain extent.