It can be the geese who go for one of them, as well as the gander. In fact, often is. My experience is that ganders fight around breeding time.
But did you say it was the one with wet, sore patches who also has blood on his beak?
What is their history, were they a 'team' already when you acquired them, or have they recently been thrown together. How old are they?
I would separate him while he mends. If you have one goose whom you're sure isn't a bully, you could put her in with him. Or a couple of ducks. Then I would re-introduce him/them by having him penned, so the others can't get at him, but can see and 'talk' to him.
I've just introduced a new gander to my two geese. Initially I had him in his own pen, as above. After two weeks, I let him out with the geese during the day, but put him in his own house still for another week overnight. Now he's in with them, they pecked him a bit for the first couple of days, but have settled down now.
Longer term, I would separate your 7 into two groups, each with their own gander, and their own house. You might need to make the separation even more significant around breeding time.
Or of course you could put one of the ganders in the freezer.