What to do when support is uncooperative

I know this has been done time and time again, but typically the thread is about the player being bad etc.

I realize I can't choose the quality of my support, though recently it feels like the support players aren't even trying to win, and at minute 1 they've already decided wether were going to win or lose.

e.g level 1 my bard engages on the braum caitlyn lane, getting poked hard, i'm varus so i match the poke and we both back off. He took way more than them but for some reason he flashes in and stuns braum with a minion. I'm not in range to shoot cait, so I attack the braum; though the cait is able to kill the flashless bard before I can kill braum and he dies.

Something to this extent happens every time he gets back to lane, and he ends up 0/4 and i'm 0/1, due to dying in one of the encounters.

Now i'm not good enough as varus to not get out-traded and die when the enemy fizz comes bot and bard is dead, so I back off and let the tower die.

At this point its 0/7, and I dont know what to do. Bard wanted to kill them level 1, and it felt like he didn't want to play any part of the game other than fight. I asked him to back off and he told me to fuck off, then told me I was shit because I couldn't kill braum.

If I stop to type to him, the cait that is already shooting me takes my tier 2, and if I dont respond he just engages a losing fight.

What is the language I need to use to get a support player to treat me as a care package he needs to guard, rather than his vigilante buddy that is suppose to come get into random fights with him for no reason.