OP, you weren't to blame. Even if you'd called him a useless twat and left him there, you still wouldn't be to blame. He chose to end his life. We can only speculate on the reasons, but there will have been many.
I think he was terrified and asking for help but couldn't explain it in explicit terms.
Perhaps that's so, but you are not a mind-reader and the responsibility was on him to ask for help if he wanted it. It actually sound more like he had already made up his mind, as he was telling you basically how to live your life in the future. If he had wanted to not die, he would not have killed himself.
As for his dad - psh. People who are grieving often have an urge to lash out at the nearest target. A father who has been largely absent is going to feel even more like doing that to assuage his own guilt. He probably also told you're BF's mum she was to blame.
BTW I went through something similar. Split with my abusive BF, went back to pick up some things, he'd taken an overdose. I got him to the hospital in time. Was it my fault he took the pills? Hell no. He did that all by himself.
More recently a colleague lost his boyfriend of 3 years - they had a stupid argument one night, both been drinking, BF went out, got on a bus to the station, and threw himself under a train. His family have in no way held my colleague responsible, because it wasn't his fault. They knew the BF had had MH problems for years. They have supported each other through their grief.
You have been carrying this burden of guilt around for 15 years, and it's okay to put it down now. Please seek help from a skilled therapist who can guide you through letting go of this. You don't deserve to spend the rest of your life feeling the guilt for something that was never your fault.