What is the right thing to do?

Hi first time poster and scared for my ex.

About 2 months ago my partner of 12 yrs. left me. It’s a long story but over the last year or 2 he has pulled away, lost sex drive, stopped socialising, pushed friends away, was never happy, got angry frequently and was unable to show any compassion or empathy even towards me and my family when my grandfather passed. He was depressed and found no joy in things. He has now hit the bottle, acting uncharacteristically (like a teenager) and only looks at today and not the future. He says there is no us right now but won’t commit to us being done for good. Says he doesn’t know who he is and just isn’t happy.

Since our separation he has started to follow in his mum’s footsteps. She passed away early this year from her illness. She had several mental conditions but the main being Borderline Personality Disorder. I never thought of it while we were together- maybe I was too close or didn’t want to see it, but since being apart I have seen his behaviour mimic hers over and over. He is extremely reluctant to get help although he has said he would a few times it never eventuates. He is not the man I know him to be. He has changed before my eyes.

My question is... what is my role? He has very little family to support him- he relied on mine for the last 10+ yrs. and he keeps his mates at a distance. No one can see the whole picture- he manipulates his life so no one ever sees what’s wrong. I haven't reached out to his mates or brother as I’m scared to push him further away. I still love him and care about him and if I’m right and he is sick, it is his worst nightmare. He has always been so anti mental illness (for himself) due to seeing his mum suffer and have her life implode due to BPD. She got divorced, could no longer work, in and out of inpatient care, abuse her sons, became an absentee parent, abuse medication, alcohol and drugs, couldn't handle day to day tasks, hated being in public and relied on her sons for pretty much everything. He has made it his life’s mission to not be her and here he is denying anything is wrong which is exactly what she did at first.

Do I reach out to his mates or brother or his step dad who divorced his mum when she did what he has done to me to him? He's pushed me away and made it clear he doesn’t want my help. Although wants to be friends (but treats me horribly in the process). Meanwhile I’m feeling guilty and concerned (on top of heartbroken and depressed) like I'm watching someone wanting to jump off a bridge and not say a word- just letting them do it. My friends say it’s not my place and don’t understand BPD and he won’t talk to me.

What is my responsibility here? I love him and want what’s best but do I have to let him go and do it his way or am I responsible to speak up or act. I am genuinely concerned he has BPD or at least something along those lines.

Many thanks