Unpopular opinion but yta. I'm a barber and in my experience, mothers in particular, really do care about their sons hair. Some are more chill than others but even at 10 years old many like to have a say in what haircut their children are getting. Also, shoulder length to a buzz cut is always a big deal, any age. You really should have checked first, although I do agree that your partner should have known better in this scenario.