You don't have to seperate a horse to teach it to be alone, you have to build its confidence in its mind and stability in its body to teach it to be more comfortable outside of the security of the herd.
This is true, the problem is most people don't know how to give the horse that confidence, particularly when the horse is doing everything in its power to get back to the herd, making an unsafe situation.
Then there's the issue of taking a secure horse away from the other horses and having the other horse/s go berserk when their confidence blanket is taken away and endangering themselves.
The easiest, quickest, most effective way to teach horse AND
human is to separate completely and let horse and human build there relationship w/o inference from another.
The advice one gives needs to be appropriate to the situation, but mostly it needs to be appropriate to the person
who's having the problem. If the OP could instill confidence in this horse while letting it remain with the other horse, they wouldn't be here asking the question. You also have to keep in mind the OP is working with a young, green horse who's been a stallion until very recently. We've got hormones, adolescence and ingrained behavior, perhaps prior poor handling and training to deal with.
You're giving a false sense of solution here. It can take months and even years, depending on the horse, situation and past for this horse to gain that type of self-confidence you're talking about. You have no idea what this horse has been through or what his base personality and temperament is. You don't know if it's his hormones talking, or if he was always the baby in a herd who was allowed to jump and breed whoever was in the vicinity.