I would try do some round pen work and try keep him focused on you not trying to go back to the herd
I agree with all the answers except that there is nothing wrong with leading in a bit or control headcollar if it improves YOUR safety. If you can't get your horse away from his stable or herd safely, it will be hard to do any ground work with him!
I would recommend a heavy dose of patience. Maybe don't always lead him away from his herd but lead him around them (wear a hat) and ask him to move his quarters or to back up in the field. Then progress to moving away from his friends. Good luck!
Rearing is simply your horses way of telling you that he does not want to move forward. In other words, its them refusing to move, except on their hind legs. It's perfectly natural for a horse to have anxiety - especially since they're prey animals and naturally, they will want to be with other horses to feel safe. Using a chain, chifney, etc will only cause pain to your horse and will create more anxiety. Please do NOT do this; your horse will become fearful of you and more disrespectful. Although it is a quick fix, your aim here is not to use pain against your horse to make him do as you request. Instead, it is to ensure he feels safe enough around you to not want to desperately be with other horses all of the time.
As Charlotte said, groundwork is absolutely essential. You gain a horse's trust and respect through consistent groundwork, including lunging for respect, backing, yielding the hindquarters and forequarters, etc. You need to gain control of his feet rather than his head. If you have control of his feet, you have control of your horse.
If you can feel him becoming nervous when you're leading him away from his friends, keep him busy and grab his attention (not through treats - you don't want him to believe you're a treat machine). Use the concept of pressure and release (I recommend watching Rick Gore's videos for this) to keep his mind occupied and worrying more about you rather than being with his buddies.
I hope this helps!
You can try lead your horse in a chifney
Hey! So the main problem here seems to be your horse is heard bound. Ground work is very important, but also, ask someone who owns a horse in your pasture to go on a ride with you away from the field. Let them know that you’re not leaving their friends. Ground work is super helpful and i really reccomend it ! Good luck