This is good stuff.I would strongly encourage you to apologize to your son, to the degree that you feel comfortable. By acknowledging his anger you'll help him move past that. If you don't acknowledge his anger, he's going to stay angry most likely, and that will impede your ability to have a relationship with him. You did play a part in raising him, and it's clear that you feel regret and some responsibility. Just tell him that.
Anger and resentment serve a purpose. They are a normal and healthy response to threats to our well-being.
Serving a childhood sentence in this miserable CULT certainly qualifies.
The son is entitled to his feelings and they need to be acknowledged and validated before he can choose to start moving past them.