The best option is to file a motion to modify custody based on the fact that the mother will be moving to another state. If your current order does not address moves by either parent a motion will need to be filed to update the custody arrangement / visitation schedule. You could be awared primary custody with the mother having visitation. There are a lot of unknowns so my answer is general.
This information provided is general in nature and does not create an attorney client privilege. Please consult with an attorney to get a more detailed and specific answer to your legal matter.
Technically, the mom could leave with the child so long as there is nothing in the custody agreement speaking to out-of-state moves by the parents. If your agreement is silent on that point, and if the mom moves with the child you could still petition the Court for an order requiring the child to be returned to the State and for the Order to be amended to deal with out-of-state moves, custody, visitation, etc. If that is your goal. Keep in mind that the jurisdiction of the Court flows from the residency of the child, and that a child's residency in a certain location for a certain amount of time will vest the new location with jurisdiction to hear this type of matter. So, if this is to be a "permanent" move, you could be facing a situation where you'll be required to travel to a different state to address matters dealing with the custody arrangement. Just a thought. Best wishes.