My boyfriend stopped washing his hair about 2 years ago

Is it gross to only wash your hair 2x a week with straight apple cider vinegar? He is trying to fight hair loss and read somewhere that shampoo makes it worse. He refuses to use shampoo anymore, and it's been that way for 2 years. His scalp does not smell good, even after "washing" it. He wears hats all day every day and works in metal fabrication. His side of the sheets are stained and his pillow (brand new) is already stained through the pillow case, but that could just be because he works in metal fabrication?? Or sweats at night??

Please help me get over the fact that this grosses me out (yes I've told him this and he doesn't care) or explain to me that it is cleaning his scalp/hair so I can get over my discomfort and move on! Thank you in advance!

Update -

Hey all thank you for all the suggestions and comments. I’ve read them all and digested them. Firstly, we’ve been through a lot together, making the solution of “just leave” not my first thought because I want this to work. He’s a good man that has been misled by information, I think so anyway. We had a conversation the night I posted this and it didn’t end well, at all. He gave me the silent treatment after I brought it up and just said “I’ll just shave my head” over and over any time I tried to talk to him about other options. I never said for him to shave it, but he was insisting, I’m guessing to stop it from even being a conversation. He was mad and being not nice to me at all when he did actually respond. I slept in the living room because I wanted to give him space. In the morning he was leaving for work and he said bye and I said that’s all you have to say to me? Then he proceeded to say some hurtful things to me, that I only care about my own feelings, not his and I don’t want to help his problem, but to help myself. When he said that I got extremely sad and upset because he should know me better than that after the length of time we’ve been together. I told him that if that’s how you view me then why am I even here because that’s not how I am at all and he knows this. I think his ego was hurt? I told him that I genuinely do not think what you are doing is helping you nor is it healthy if your hair and scalp smells, and it’s also affecting me more than I’d like so I was hoping we could find something you are okay with that also makes me feel better about the situation. Then he left for work. That night we talked about it again, wasn’t getting anywhere as he refused to consider using shampoo, fine whatever as long as it doesn’t smell idc what you do or use. But it got into an argument about my motive for the conversation again, and only wanting to help myself. Only caring about myself, which I again explained it is about me but you as well. It’s about both of us. Something you’re doing is bothering me to this extent and I’d like to try to find a compromise. He could not accept that as the truth for whatever reason and I have always been a very honest person. At one point I just cut him off and said “why are we even arguing about this? You need to wash your hair, it smells and it’s gross and this shouldn’t even have to be more than a simple conversation, idc if you don’t use shampoo so long as your hair doesn’t stink and leave residue. What are we doing” then we sat in silence for a while, I was fuming. After calming down, he settled on trying baking soda paste with white vinegar, as some of you had suggested, and if that doesn’t work he said he’d try a conditioner cowash. I am okay with this solution, but we didn’t get there without a lot of unnecessary arguing. I’m still pretty upset but I’m letting him try to keep his word on what he said he’d try. If he doesn’t then I’ll reevaluate. Thank you all again for your help and comments, they helped more than you think.