Also ...
I would suggest that your son may not have a problem with anger management as much as an issue with sensory input mitigation, prediction, and control. AS tend to melt down, find themselves unable to cope, more than actually experiencing anger. Which is manageable by learning the triggers and developing coping techniques. If the father and his wife have made no effort to get to the core of the meltdowns, they can't expect the child to.
My son cannot deal with unexpected changes in routine or outcome, "playing things by ear" on an outing, too many people, too much noise, and so on. Knowing these things helps me plan his day so that we don't encounter them any more than absolutely necessary, and also lets me know when to be extra sensitive to potential warning signs of overload. Before I figured all that out, I would have said my son had an issue controlling his temper, but I now know it isn't his temper, he's actually a really calm kid, it's his AS.
Yes, many AS do have anger, but it has often come from years of being misunderstood. It comes from the frustration of learning by life experience that nothing you do or say matters, it will always go wrong, so why try? I can't say if your child has had that experience in his father's home, but if he has, then it could be the source, and it will take a lot of trust building to overcome it.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).