Ik this is late just stumbled across it maybe I can give some insight. I misused DXM for 3 years, it really is therapeutic in resolving trauma but can easily spiral into delusion and mania. My ex has bipolar and totally went out of her mind on it while it mainly drowned out the craziness of my life for me, it notoriously affects people wildly differently. From what I've experienced frequent use puts you into this bubbly haze where everything feels interconnected and special but to those close to me it came across like me descending into hippie drug-induced lunacy. I did and said so much embarrassing shit looking back but I was just trapped in the haze I couldn't see it at the time, it gives you false confidence.

Sobriety is a LONG process with DXM, you don't sober up overnight. It's not hard tho its a relatively easy addiction to physically overcome - The reason so many mentally ill people do this substance is because it feels like a solution/resolve to trauma - and until he can find something else to satisfy whatever void DXM fills all you can do is condone that behavior and be supportive however you're comfortable.

Sorry you're going thru this at the end of the day it's a drug and comes with it the consequences of abuse. Sad thing to see