When a mother is too attached to her son it's called enmeshment or codependency.
Enmeshment and codependency are terms that describe a relationship in which boundaries are blurred and the parent's sense of self is overly tied to the child's well being and actions.
And in some cases, it can also be described as smothering or over parenting or even in extreme cases emotional incest.
Enmeshment is the lack of healthy boundaries between the parent and the child and is where the parent's emotions and needs are also deeply intertwined with the child's.
The parent may also feel responsible for the feelings of the child and the child might even feel obligated to meet the needs of the parent or parents.
Codependency is a relationship in which one person's self worth is dependent on the other person's needs and approval.
And in the context of a mother and son, it could manifest as the mother needing to be needed by her son and often at the expense of his independence.
Over parenting is another term that is used to describe the behaviors of parents that are overly involved in the child's life and often beyond what is age appropriate.
Smothering is a term that describes the act of a parent being overly attentive and controlling of their child and it can potentially hinder the child's natural development and independence.
And emotional incest is what occurs in extreme cases and is a mother's excessive attachment that can be described as emotional incest in which the mother relies on the son for emotional support in a way which is not appropriate for their relationship.
The terms all also point to an unhealthy dynamic in which the mother's excessive attachment can negatively impact the son's development of independence and healthy relationships.