I am so glad you have reached out for some support here, and that your partner is going to be getting some support and help at this time. I am so sorry this illness is putting such pressure and strain on your relationship, that is really hard.
You mentioned that he accuses you of cheating and that it turns to a heated argument, can I ask at this time that instead of engaging in the argument you calmly, yeah easier said that done, ask him if he can explain to you what behaviors you are doing that would indicate you are cheating, when he replies once again try hard not to engage in argument and self defense as you know from the past it ends in a fight. Let him speak, let him say what the things are, they are probably going to sound totally unreasonable to you but hear what he has to say. Keep in mind he is mentally unwell so things he sees and says may not be rational or true, as is his accusations, but let him speak. Ask him what you can do to ensure him that you are not cheating on him, what things will help him through this time, that you are on his team and you love him.
I have no idea how difficult it is to support a loved one through this illness and to see the person you love suffer like this, my heart goes out to you. Can I ask if you are getting some support too, it is so very important for you to take care of your mental health too Mousy.
I am no professional so I am not sure if what I have said helps, but we are here for you, so huge hugs to you.
Hope to chat some more.