Childhood attachments are complicated. Emotional skills develop in the brain quicker between the ages of 6-18 months and in the late teens. The right side develops the intuitive and emotional skills which are needed for relationships, and for the empathic understanding of another person’s feelings. The way children are raised is not as influential as once thought and some children are just born more susceptible to insecure attachments than others. With susceptible children serious risk factors include having a parent who was raised in care, having parents who are chronically neglectful, have a drug or alcohol problem, are abusive, violent or criminal. Other childhood risk-factors that are not as serious include parental depression, young unsupported parenthood, and family breakdown. There is a cocktail effect so 3 or more low risk factors combined become high risk.
Traditional 'talk' or 'play' therapies do not work with children with reactive attachment disorder because such therapies depend upon the child's ability to develop a trusting relationship with the therapist that the child is unable to form. Therefore parenting must be very structured and therapy, which always includes parents, might be re-parenting, role-playing, Gestalt Therapy, family therapy and general psychotherapy.
However, I would be cautious because school staff do not have the skills or diagnostic tools to determine your son's condition and there might be other explanations for his behaviour