The most crucial element of dating someone who has a mood disorder is that they are managing it through some form of therapy, whether that includes medication or sessions with a professional or (usually) both. If they are unwilling to do that, there is nothing you can do, your only realistic options are to accept poor treatment due to their condition, or leave the relationship. You can't help them, it's not possible for you as a SO to give them what they need to manage their condition.

Consider that by shirking any kind of help, your SO has decided that you should suffer along with him. That's not fair or loving. A person who loves you should want to make your life better, especially when they know that they have a problem which needs to be fixed.