Sounds like he does not have another horse ?? If so AND you own the barn, how about a ‘friend’. We have 4 horses, but someone gave me a Guinea Hen and she (as she was alone) hung out with the horses ALL day. She would roost at night in the barn. I put a heat lamp up for her by her roost in the winter. Anyway, she has gotten to the point where she rides around on 3 of my 4 horses. In the nice weather, she hangs with the herd and eats bugs AND TICKS, wherever they are grazing. Just a thought. ps Guineas do not make the mess that a chicken does, are less likely to end up in our garden and are fairly easy to care for.