If there's any way you can, school him in the field with his friends or get someone to help you school him where he can see and be near them. When he's working comfortably, start adding distance to the horses and gradually work it up to being out of sight of them. It's a long process but it's the best way I've found to get over a horse being herd bound. Each time you increase the distance, increase it to the point where the horse starts to be a little more uncomfortable and then work there until he relaxes. Patience is your best friend with a horse like this.