As for his not answering your questions, this is not peculiar to him. I have heard the "I don't know" from all of mine. Sometimes they don't know, and sometimes they are being evasive. Part of it, I think, has to do with privacy.
The sadness and crying is something else. His father has gone off to work. Is that the answer? Is his father working abroad?
He might developed a depression. İ would got to a therapist beside supporting and loving him. İm sorry for him. İt should be so hard...
This little guy has been through some pretty emotionally hurtful things in his life. First when his mommy left and never came back. That is devastating to a child, even if they are not old enough to fully understand it. Then his daddy having to return to service (thank him by the way) Losing first one parent and then the other is alot for six year old to handle. Do you make it a fact to him that Daddy is going to come back? He needs constant reassurance of this so he doesn't feel abandoned again. I'm sure his acting out is emotional. Maybe by putting the focus on something else it could help him out of his cycle. Instead of telling him what he does wrong, only tell him the things he is doing right. There is alot riding on you as a parental role for him right now. Maybe this could turn things around some for you. The crying over everything? He will out grow that eventually, honestly. He must know he is loved so remind him often of all the people who love him. Really reward him when he does something right, and please let us know how he is doing. Thank you for doing your best for a lonely little boy!