The poop looks dry and scant, and that makes me think he could have an impacted crop or gizzard. Is he an "only child"? He sounds lonely. If he has no companion, and that can include a dog or cat as well as another goose, his well being could be poor and is affecting his health.
Check his crop in the morning after taking up his food and water tonight after you put him to bed in his coop. If it's full and hard like a sack of feed in the morning give him some coconut oil, abut two teaspoons to start and massage it. If his crop is spongy like a water balloon, don't massage. He may have sour crop. That would have to be treated with a vaginal yeast cream.
If he's a solitaire, I would hang a mirror in his coop so he can see his image where he is used to sleeping. A little thing like that could make a lot of difference to him, making his feel a though there is another goose in his coop with him. It could raise his contentment level.