My dad is killing himself with alcohol

He's been diagnosed with diabetes and isn't doing anything about it like, diabetics have to check their blood sugar and insulin (im sorry if that's wrong I have no idea) and he's not doing that. He's not watching what he eats and he's still drinking alcohol like it's water. To put it blankly, he doesn't give a shit.

So he probably won't live much longer.

Btw. My mom has tried multiple times to get him into rehab and even got him a bed in one a few times and he refused to go.

My mom hasn't been with my dad in 20 years. He was physically abusive to her and almost killed her. I met him when I was 14. But he's severe bipolar (refuses meds for it) and verbally abusive to me and my brother so I don't have much to do with him.

What do I do? What am I supposed to do? He's a piece of shit but that's the only dad I have.

Edit: I know I can't help him. He's been drinking my entire life. I don't know though, If i should spend time with him. I don't want to look back on this after not speaking to him even while he was dying and regret it deeply.