My 11 1/2 month olds habit?how do i break it?

Not sure if him still wanting formula is really an issue, but if you really want him to stop the best way is "cold turkey". He's only a year old - you can't reason with him, he wants something and cries when he doesn't get it. That's what babies do - it's how they communicate. If he doesn't like watered down formula, just stop. He's gonna scream an cry and make you feel horrible for a while, but he (and you) will get through it.