It is hard, but you deserve a life. You can't stop your son's drama, but you can minimalize your contact with it.
My daughter walked to and from work in the Chicago winter. She survived. Even if they "need" (usually it is more like want) a car for work, should we help them risk their own lives and the lives of others to make us feel better? I don't understand helping an intoxicated adult kid drive while intoxicated for any reason, even a job. It's unsafe and we all know that substance abuse is a part of at least half car accodent fatalities. The bus sounds good.
We worry too much about food. Trust me, your son has some money. They don't slways get it legally....drug sales, selling other things (the car?), panhandling, stealing etc. If he has small change, and if he didn't he would be guilting you about that, he can get stuff out of a vending machine. Many of us don't eat full meals at work. He works so he should have money. The shelter should give him other meals.
Please get therapy to help you take your life back. You deserve amazing golden years.