I'd provide extra feeders, waterers, roosting areas... plus places for the bird to duck behind (but not get trapped behind) to avoid the aggression while in the coop/pen. They may work through it and change their pecking order (pecking orders can change frequently), or the ostracized bird will eventually figure out how far away from the others it needs to stay, but still be able to tag along with them. You could also try getting a few more birds that are younger and gradually integrating them, so the ostracized bird has a new "gang" to pal around with that will be more friendly to him/her... plus the bullying by the others will be more wide spread, rather than just focused on the one singled out bird.