@Let it marinate i have a theory of my own. Two to be exact, don't you notice how hyeol looks like the ml while kochsu looks like the mc.
Either the two were switched at birth or that the mc and ml actually dated and had a family but then separated and each one kept one son, the ml knows about it and because of whatever stuff the mc said about souls he thinks that because of that he doesn't remember that they are both his kids.