As far as what you can do… It sounds like your doing it. Your listening, and being his friend. You already picked up on the fact that you can’t talk him out of his belief, you can only logic around them. It’s hard and you too will need some ideas on how to keep being a friend. There is a NAMI site that has support groups and education about SZ and other communication ideas.
It’s going to be a bit of a rough ride considering he’s in the onset. So much going on, so much can feel confusing. I was NOT a nice person durning my onset. I was convinced it was ALL “Them” everything was due to “Them” not me. You won’t talk him out of that. Keep the line of communication open. Stay calm and if he does start yelling at you… You have every right to simply get up and leave.
You can set boundaries, you can let him know that you’re there for him, but if he’s going to rage, you have to leave.
Look up the NAMI site and get as much education as you can. My head circus had a new “gift” almost every week during my on set and my amazing family who had been reading the books and getting the ideas sometimes knew what was happening even before I did.
I wish you luck. Remission is possible. But the onset is the hardest, most confusing part. The sooner he gets help of some kind, the better off he will be, the more cognitive and other functioning ability he’ll maintain.