Agree with stevenson about getting the horse accustomed to being alone now. I had to do this with my gelding and mare (now deceased). Of course didn't help it was mother and son, but still , buddyed up is buddyed up!
I would put the gelding in the paddock to prevent him from galloping wildly throughout the pasture, and put hay down for him to munch on. I guess my thought process was, "oh, look , you may be alone, but you don't have to share this yummy hay!'. Then I was just go off on a trail ride. Of course at first he carried on, but after the first 5 to 10 mins he'd settle down, until he heard his mom coming back to the yard.
Started doing about 30 mins, then quickly built up to 1 hour and then easily to 3 hours. The real test was when I did an overnight with my mare and left him behind. Husband told me he called a bit and then settled down. Would call again here and there but nothing nutty. I of course was calling at least 3 x's a day, because I was sure he would work himself up and colic! Silly me!
Now I'm doing the same routine with the new mare. She is very anxious about Walka being taken away from her, but she has survived for 12 yrs without him, she will live.