Most Helpful Guy
Communication, education and patience.
He has to be actively seeking help at the same time, otherwise nothing is going to work out between him and anyone. He needs therapy and medication (if necessary) and he needs to take it seriously. He needs to learn about his own disease and COMMUNICATE if he's having a "sick day" or needs alone time or communicate whatever other needs may arise for him specifically. If he hasn't already taken the initiative, open that line of communication.
You need to EDUCATE yourself as well. I know this seems like I'm putting an unfair burden on you, but believe me it will help. Read books, articles, talk to professionals, etc. Learn why he might behave the way he does and try to understand his feelings/delusions/compulsions/etc.
PATIENCE should be obvious. Even if he's doing everything right, most mental illnesses are life-long deployments, and a sufferer is going to have some bad days. There are going to be times where it is hard for people he loves, but as long as it isn't abusive behavior, try to be understanding. Sometimes it's hard for depressed people to get out of bed. Someone with OCD might find it hard to do anything in a timely manner. The specifics are variable.
Also, you have to take care of yourself first. I'm mentally ill, so I know from experience, I can be a fucking burden, and I would feel awful if that was someone else's problem too. Have boundaries, communicate them, and look out for yourself first and foremost.
Most Helpful Girl
Depends on the mental illness.