Communication through the kids - how do I stop this?

My ex keeps asking our 8 year old to relay messages to me instead of just texting me directly. Stuff like "tell daddy I need to change the schedule" or asking the kid about my personal life. I don't want our son in the middle of this. I've asked her multiple times to contact me directly. Do I need a custody agreement amendment or is there something in family law about this? Just seems unhealthy for the kid.