Wait, the 4yo horse has never been weaned?!
I witnessed a situation like this once before with a 9 to mare. Basically one day they sold the mare and trailered her away from mom. Mom was very stressed and then depressed until we found her another companion. younger mare was the same way- calling, running around, it took her about a week to calm down and settle into her new herd.
If possible, I would put them in separate pastures because until you have them constantly separated it will be next to impossible to train him out of this. It's his mother, it's a natural thing for him to want to be right with her all the time.
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