Welcome to the forum
I think generally speaking it's a pretty common problem with a lot of horses. It's actually one of the first things I work with whenever I buy a baby for training.
The problem with a horse that is herd bound is that they long for safety. A buddy out in pasture is a horse that they trust and therefor want to stay with. What you need to do is show that horse (him/her) that you are just as trustworthy and that going away from the herd is A-ok. It's something that takes weeks if not months to develop. I will try to explain this one to you by telling you about my monkey.
When I first got Cobalt he was very young and didn't know much of anything(only 8 month old). I started out by catching and tying him inside his field, brushed him, played with him then fed him at the end. Did that until we had a routine. Once we got over that, we started to go for not just a grooming in his field but I'd take him for a simple walk down the driveway and then back. He didn't like it, he would call his mare, he'd throw temper tantrums(which were quite funny) but I ignored it, continued talking talking to him and finished the walk. Again did that until it was routine. From there I added more and more. He eventually spent more and more time with me away from his buddy and he realised that it was always ok. I never asked anything of him other than come with me for a walk.
Take your time, spend time with your horse, build a bond and things will improve but don't rush things.