Two specific factors:
He is not promoting the relationship with you and your son. He is actually alienating the relationship.
He has changed the child to public school and the child's grades have been affected.
The child wants to live with you. The child has to be 10-12 with a maturity level to choose the parent he wants to live with and the judge will 'consider' the child's choice if you are stable and can provide a loving home.
You are still with the same partner and raising your children. I am assuming your partner has a job and no criminal record. If this is the case then you have a good chance to gain custody. The child will have to testify his wishes to live with you and that his father is alienating the relationship with you.
Best Interests of the Child Factors:
As used in this act, “best interests of the child” means the sum total of the following factors to be considered, evaluated, and determined by the court:
(a) The love, affection, and other emotional ties existing between the parties involved and the child.
(b) The capacity and disposition of the parties involved to give the child love, affection, and guidance and to continue the education and raising of the child in his or her religion or creed, if any.
(c) The capacity and disposition of the parties involved to provide the child with food, clothing, medical care or other remedial care recognized and permitted under the laws of this state in place of medical care, and other material needs.
(d) The length of time the child has lived in a stable, satisfactory environment, and the desirability of maintaining continuity.
(e) The permanence, as a family unit, of the existing or proposed custodial home or homes.
(f) The moral fitness of the parties involved.
(g) The mental and physical health of the parties involved.
(h) The home, school, and community record of the child.
(i) The reasonable preference of the child, if the court considers the child to be of sufficient age to express preference.
(j) The willingness and ability of each of the parties to facilitate and encourage a close and continuing parent-child relationship between the child and the other parent or the child and the parents.
(k) Domestic violence, regardless of whether the violence was directed against or witnessed by the child.
(l) Any other factor considered by the court to be relevant to a particular child custody dispute.