I think it's very easy for me as a non Muslim women to say 'screw him' and do what you want, because it's hard for me to imagine a guy being so bothered about me covering my hair.
But trying to look at it from a different perspective, if it was agreed when you married, and he believes it's indecent for a woman to not wear hijab, then that's more akin (to him) of me wearing something my culture would consist indecent. So perhaps deciding I'm going to go about topless from now on. I'm not saying they are the same, but in the context they may feel equivalent. In that context, you'd still have the right to do so, but his objection seems more understandable.
If you still love eachother, I hope you can both find a way forward. Ultimately, if his desire to be devout, with a devout wife, is stronger than his love for you, then you're probably doomed.
It could be that a compromise could be formed, where you don't wear it most of the time, but do at family events, for example. But equally, you don't have to do that unless you are content with that compromise. In marriage, people still grow and change, and the question is whether your grow together or apart. A change in a shared religion will pull you apart, but what that means is upto both of you together.