kingyo no fun
people who always follow or tag along with a particular person
This idiom originates from the fact that when goldfish takes a poop, it often remains attached to the fish as if following it. It is usually used as 金魚のフンのように or 金魚のフンみたいに, both meaning “like a goldfish poop” (often sarcastic). フン can be written as 糞 in kanji as well.
This expression might be getting a little old-fashioned; a more famous idiom which has a similar (albeit not the same) meaning is probably 虎の威を借る狐, which literally means “a fox that borrows the authority of a tiger” and figuratively “people who behave pretentiously behind someone strong or important”.
He is always with his boss like a goldfish poop.