Hi tas, how are you doing? it sounds like a really difficult complicated situation, your son is lucky to have people like you and his gran who care about him. Fom what you say he is really poorly, and that might be one of the reasons he can't see it - mental illness plays tricks on you.
Is he getting support from the mental health team or has he declined help? It sounds like he might have done from what you say - just wondering if he is on any meds. Are you aware of something happening that caused his recent decline?
I can't offer any magic tips, but do have a look at the Mind website - it has lots of info about the different conditions and how to support others. https://www.mind.org.uk/information-sup ... eone-else/
Are you looking after yourself too? If it gets too much, it is ok to ask for help - your GP will be there to listen and may be able to signpost support for you, for example the Carers Trust https://carers.org/article/alcohol-drugs-and-addiction