I advise you to proceed with caution. It sounds like the man you love is not stable enough right now to deal with marriage. He needs to stabilize his illness first, and the best way to do that is to see a psychiatrist regularly and take all medications she prescribes. Offer to go with him if he is too paranoid to go alone.
Bear in mind that even a marital relationship between two mentally and physically healthy persons can be stressful. A disability just compounds the stress. You don't want to get yourself into a position where you must be his caretaker because that is never a good basis for marriage. He has to be able to give to the relationship as well as take from it. And if he refuses treatment and is non-compliant with taking the medications, I would urge you to call off plans for marriage and just remain his friend. He obviously needs caring friends.
Carolyn