Generally, I think it is better for a child to have intermittent contact with his mother than none at all. He does have a mother who seems to have some feelings for him, although she may be unfit to care for him. Children who are cut off from a parent often develop unrealistic fantasies about that parent. I think it is better to allow supervised contact and to periodically have age-appropriate conversations with a child about what the child can expect from visits.
In this case, the mother is troubled and addicted. She isn’t able to be the mother the child deserves. “Contact” does not mean that the child should be left in her care. You already have evidence that she doesn’t take care of him when she takes him off by herself. “Contact” does not mean that the father should not assume primary custody. The mother is apparently unable to keep her child safe or to be a responsible adult.
I suggest that the father talk to a family therapist about what is in the best interests of the child. He should also talk to a lawyer about how best to handle the situation, hopefully without becoming so adversarial that the mother vanishes completely. Hopefully, she will eventually deal with her addictions and mental instability and be able to be a positive presence in her child’s life.
I wish everyone well.