Since you mentioned you're in the Netherlands, this made me think of an article I read recently by Willem H.J. Martens, director of the "Kahn Institute of Theoretical Psychiatry and Neuroscience" in Utrecht. The article I read was "Emotional Capacities and Sensitivity in Psychopaths". It states that although psychopathy is characterized by emotional deficits, there are areas where they can be hypersensitive and others where they may achieve normal emotions under specific circumstances. The article has all of its sources cited and is well researched. He has written many articles about therapy for psychopaths as well. It might be worth researching some of his articles or contacting the institute.
Reading the article gave me peace of mind. I've had trouble accepting my ex was just a psychopath and there was nothing I could have done to have affected a better outcome for the relationship. The article made me realize HAD I KNOWN she is a psychopath, I MIGHT have been able to identify things in her childhood that lead to hypersensitivity, been constantly cognisant of them and avoided them in the present, as well as worked hard to do things that brought out normal, healthy sensitivity. The massive scope of that task, a lifetime walking on eggshells avoiding hypersensitive areas while constantly having to WORK to bring out NORMAL emotional sensitivity, yet always bearing in mind psychopathic traits knowing at any time she might regress has made me finally able to let go and want to shout, "Allelujah!", I don't have to live my life like that!!!
SO, it seems there might be things you can do or types of therapy to bring out more healthy emotional functioning. But, no matter what, I would always be vigilant and watch my back knowing the psychopath could and very probably will regress. Willem Martens could be a good place to turn. Certainly if there are types of therapy that may at least help your son, something is better than nothing.