Let me tell you a story about a friend of my family. He is like your husband, but doesn't have a wife. He could NEVER throw away his mom's stuff. Three years ago she died in a nursing home at the age of 91. Her stuff is ALL OVER his house. It's in boxes and laying around, in the middle of the floor, stacked up against walls, everywhere, from floor to ceiling. He has two rooms in his house without her stuff - his bedroom (astonishingly) and the bathroom. It is truly depressing to walk into my friend's house.
You simply do not owe your mother-in-law or your husband in regards to keeping all this stuff. You don't. It's your home, too. Your husband isn't the one stressing over what to do with all of this. He doesn't deal with all the logistics of where to keep stuff, how to organize it, etc.
You need to just decide what you will and won't keep and just deal with it on your own. No discussing it. If you know something is extra special to him, keep it. Display it. But don't keep junk and don't keep things that have nothing to do with your husband.
Clutter is subjective. One person's clutter is another person's comfort. If you are a "minimalist", decide how much you can take without it making you constantly anxious and unhappy. And don't let it get to that point. That's a way of making it so that you can control it, without letting it control you. Be sensitive to his comfort level without letting it veto yours. You are the one who has to manage stuff, so YOU get to make the decisions.