Geese unless kept in an area that has high worm load are generally worm free because of their eating habits.
He could have worms but it sounds like a dietary issue, not enough B vitamins is usually the cause of balance issues.
It’s great that you switched them to flock raiser, layer feed doesn’t have enough nutrients in it for geese and ducks.
If you can find durvet liquid B complex, Rooster Booster poultry cell, and a 12 ml syringe at your feed store start giving him both of them.
3ml to 6ml of the poultry cell once daily, 3ml to 4ml of the b vitamins twice daily until you see improvement.
https://www.backyardchickens.com/ar...dications-to-all-poultry-and-waterfowl.73335/