My ex is a felon and was arrested for possession of a fire arm in the home. He has custody of my son. Is this child endagngerment

If the gun was not secured (gun lock, stored in an appropriate locattion, etc.) or otherwise was not kept or maintained as required under your state laws (other than being possessed illegally), then you probably have a good argument for child endangerment. If the gun was secured, then you would have to argue that the fact that your ex was found to have illegally possessed a firearm constituted a material change in circumstances, such that the best interest of the child now warrants a modification of custody in your favor. This would probably be an uphill battle, unless there is a specific statute in your state enumerating these particular circumstances as a ground for a child endangerment finding.  You would have to paint a picture where the simple act of possessing a firearm illegally posed a threat to the health, safety or wellbeing of the child.  The obvious argument would be, "well if he brazenly disobeyed the law, then what would stop him from hurting the child."  The problem is that a judge could simply respond, "he was a felon at the time of the initial order, so the fact he has illegally possessed a firearm is not exactly a change in circumstances, at least since the act of possessing a firearm alone, without more, does not suggest a threat to the child."  In order to get around this, you want to have hard evidence that the father not only possessed a gun when he should not have, but was also suicidal, bipolar and compulsive, a habitual hard drug user such that he could fire the gun while high and not even notice because of how high he gets, etc.  This would all require some psychiatric or medical testimony from a treating physician or expert witness.  In other words, you cannot just go into court and say these things, you need a credible witness with firsthand knowledge.  And, you must emphasize that any such circumstances are a "change" in circumstances because they were unknown or not present at the time of the initial custody order. Unfortunately, I am unable to provide representation because I am a Maryland child custody lawyer.  You should absolutely contact a family lawyer licensed in your state before proceeding further.