Ugh. I feel for you.
I've seen that in a couple of situations in our family. I grew up with a mother who went through periods of depression and who had the up-and-down behaviors that are similar to bipolar disorder. She was never diagnosed so I can't use that as a definite cause, but I'm saying the behaviors were there. Nothing I did was ever good enough - no grades, no achievement, no nothing. She always could o it better, or could have done it better if she'd ever had the chance. Yes, it affected our relationship and my self-esteem until I was old enough to get into counseling and see her for who she was. I learned to forgive her, but not trust her.
My stepkids grew up with an insecure mom who retreated a lot from life's stresses. She took long walks, leaving pretty young kids home alone to function with an oven. If she took them through a McDonald's drive-thru, she often forced the kids to order because "I don't want to talk to those people." One kid now does all the care taking of her mom (emotional - there are no health issues) and the other has become so dependent (needing money all the time because she lost everything with her abusive, gang-banger, drug dealer husband). The first child is in an emotionally abusive marriage (stalking, accusing, and one serious beating), and the second is now in a domestic violence shelter. We were not able to intervene much because, at the time, the courts always sided with the mother, who always blamed my husband and me for her stress.
My husband had a very rare tumor called a paraganglioma (endocrine tumor, non-cancerous) that produced stress hormones on an epic scale. His anxiety level increased but it was treated as a mental health issue. Only when his BP skyrocketed and heart symptoms developed, but he was lacking other signature symptoms, did they search for another cause. This tumor is the sort of thing that doctors study but rarely ever see in patients. But the result was the type of behavior you describe - blame everyone else, for big stuff and little stuff. Example: we were in the kitchen on either side of the dishwasher, with him at the sink and me collecting various dirty dishes and putting them in. I left the room, and 10 minutes later came back to him putting all the dishes away and showing me one that didn't get clean. I said, "But they're dirty - they haven't been run off yet." And the yelling started because I hadn't told him these dishes were dirty. Irrational.
Maybe your ex finds the blaming is the only way he can have some control over a situation where he has none? Or where he feels he has none? The fear of "what could've happened" takes over the ownership of having caused some of the problem, or all of it. Helping your son see that, in a warped way, his father's love for him took over and came out as anger.
You have your son in counseling, and you know you have these issues with his father. He's also at the teen years which add hormones and whatnot to the mix, plus he's still getting used to your separation from your husband. You have older kids, so you know that kids sometimes feel persecuted. And the guilt-tripping over not having a phone is typical.
I guess the only advice I would have is to work with the therapist about how much you call this "Dad's stress" - because everyone is stressed, at least some of the time, but not everyone responds with outbursts and blaming. So you might elevate the language a bit to "Dad suffers from anxiety" or something similar (and I realize you don't have a diagnosis), to let your child know that this is a condition that causes extreme reactions that are illogical. It's not okay. Dad's a grown-up and you have no right to force him into seeing a doctor, but it's not your fault or your kid's fault. It's okay for a kid to learn that a parent can be wrong. You don't want to blame Dad's upbringing, because that may affect your son's relationship with his grandparents. And it just perpetuates the cycle of blame. Perhaps there's a way to help him detach more from his father at these moments, the same skills one needs to walk away from other situations he'll encounter (name it: kids using/offering drugs, underage drinking, relationships that make you feel bad about yourself, unreasonable bosses). Part of the pain of growing up is seeing people with their warts and faults - it sucks, but it's part of the maturing that will also bring many new privileges, experiences and insights of a positive nature.
I also think you're doing great work helping him brainstorm that, while he has no ownership in the problem of being left there), he does have some ownership in not using some critical thinking skills to get out of it. The answer is not just "I need a phone" - he would need those same skills if his own phone had no signal or no battery, or if he left it at home or dropped it in the sink.
Wishing you peace and strength.