I'm sorry to hear about his condition. I hate seeing people who have addled their brains with drugs, there are too many in my life.
Psychosis is exactly what the drug
Zyprexa is for. It's active ingredient is a chemical called olanzapine, which is known to cause weight gain and increased appetite. He should not drink alcohol with his medication, since it will increase the occasional side effect of drowsiness that comes with olanzapine, and may cause his breathing to stop.
You should tell his doctor about his weight gain - maybe there is a different drug he can take or a certain treatment for him that will stop his weight gain. You should talk to your doctor about any other side effects.
Jesus Christ will help you through. -Betsy