Yes, he most definitely is herdbound. Especially so since he's in a new place and the other horses represent security to him.
You sound young and inexperienced, and unable to properly handle a badly herdbound animal. If you don't have a trainer or at least an experienced adult to help you, you need one.
Since she hasn't replied yet I will, we are bestfriends in real life. :)
She is rather experienced and her Mum is a riding instructor.
Infact, her Mum teaches students to ride and show horses at a high school. :)
She has been riding since she was very young, attends pony club & shows. I know this probally means nothing to you though, by saying that, showing and pony club don't really mean anything to me. (I'm not saying they don't mean anything to you, because they might mean alot!).
By the way, she only got Stardust this year. :)
Get the bit I ride Aussie with, it should help alot, that one you have at the moment was not working for him & I.
If you are harsh on his mouth, he should learn to behave since they don't like the harsh pulling.
If you can't turn him away from the other horses, make the reins very short and tight then turn. If it does not work try turning him in sharp circles with your reins tight and short still. Still use your legs though! :P
Hope he gets better! :)