when things have calmed down a bit, ask why he thinks that?
talk to him about why he thinks he would be anything to do with the end of your relationship, why would he blame himself like that?
my guess is that he'll raise the subject of his own behaviour.
this is your oPportunity to ask him what's behind that, why does he think he has to do things the way he does, ask him about his thoughts, what his feelings are.
reassure him that absolutely the end of your relationship is not of his doing, but that this ending will give you and him the space and time to talk and work through how he's feeling about life etc, and why he's having issues at school etc.
tell him that you too are sad about DP, but that if something good comes out of it, your closeness to your son, then there's the silver lining you both need.
your dp caused this rift, not your ds. your ds needs you now, be there for him and it'll take your mind off dp!
you'll be fine, your ds will be fine.