Which word can't be a synonym for the word 'bossy?' You mean ' can be ' ?
Bossy > domineering is the closest to bossy . Boss dominates all in office .
tenacious > having a quality of never giving up on something while you are determined to do it . Their tenacious attacks on the enemies.../He is a tenacious boss : he will try all he can to keep profits up, even though its a challenge this year
dictatorial and authoritarian are similar in meaning : you cant say no to them coz they have absolute power
So, the best word are domineering , then dictatorial or authoritarian , depending on the extend of his bossy behavior . Opposite > submissive boss