The standard is frequent and continuing contact with both parents and custody orders which are in the children's best interests. If Dad's condition poses a danger to the children or being with Dad impacts the health, safety and/or welfare of the children, then that would be a proper basis for modifying a custody order. ADHD in and of itself, or the fact that Dad's wife takes care of the kids would not likely be a strong basis/argument for changing the current custody order.
You'll need to review the current custody order, and ALL the facts, with an experienced family law attorney to figure out the best way to address this issue. It is unlikely that Dad would lose all custodial time with your son, but you do not say what the current custodial arrangement is.
Not knowing the current custody order makes it hard to give an informed response. There should be communication between you and your ex to facilitate the best interest of the children and to give you a better understanding of what limitations your ex's disability places on him in caring for your son.
If the disability prevents your ex from providing a safe environment for your son there may be a basis for modification of the order. The courts want a child to have both parents in their lives but they have to be able to care for the child.
Note this answer does not constitute legal advice, and should not be relied on. Each situation is fact specific and court specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship
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