Surely at 10 if his dad had said would you like your hair cut and he wanted to grow it, he would have said 'no, I want to grow it' - basically exactly the same as what he said to you?
Actually in many circumstances that's simply not the case I'm afraid. The balance between parenting styles of a mother and father does split when the children lead separate lives with each, I have witnessed.
My DS is 12 and with me lives in an environment now where he communicates wishes thoughts and feelings with me, where we discuss and negotiate.
With his father it's now just in two ways - crazy hedonistic fun and authoritarian. So if his father said "you're getting a haircut" I think my DS would go along with it against his wishes, because dad said so.
And when he switches from house to house he changes his personality a bit to fit the mood it seems. 
Sorry you are feeling this way OP. How does your DS feel about it? I'd be more annoyed that his wishes weren't taken into account. And of course of you have a strong relationship with your DS, you'll hear how unhappy he is, if that's the case, not his dad.
It's another fallout pattern that you have to deal with in separation. 