In Missouri the Family Court judges use the standard of what is in the best interest of the child. From your description, it does not sound as though your ex has acted in your son's best interests. However, the ex still does have custody rights. You will need to have a judge determine if he should have supervised contact with your son. If supervised visitation is successful, it may be the Judge wants to move to unsupervised contact. When your case is heard at court, because you have alleged abuse and neglect, the Judge will have to appoint a Guardian ad Litem. A Guardian ad Litem is an attorney who is appointed to represent your son's best interests. The GAL will be a strong advocate to procted your son.
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