Other goose owners please correct me if I am wrong but I have always been told that spinach is bad for the geese. I go to my local produce market and they give me boxes of Romaine, Red Leaf, Broccoli leaves, Califlower leaves and greens but I always ask them not to put spinach in the mix. Mine won't touch waterfowl feed but love corn, scratch grains and even wild bird seed. They also graze in the yard and pond.
I'm not saying that's what killed your goose only that this is what I have been told about spinach. I'm sorry to hear you lost him, it's good that you put others in with the female, they are social birds.