One thing there are plenty of for the homeless are others in the homeless community who know the ropes and places to eat for free. I wouldn't worry about that too much. Often they like to call to stay they are "starving." If your son does that and asks for money DON'T DO IT. Bring him bread and peanut butter (large jar), but don't give him money. He will use it for drugs, not food.
Eating is about the easiest trick a homeless person learns about. There are sleeping shelters too, but they require rules to sleep there, such as you are sober. And you have to be there at a certain time. I worked at a really nice shelter at a church. The church ladies always cooked a great meal. The clients got free train tickets to the next night's church that offered a mattress, warmth, and food. Some didn't keep coming because they didn't want to stay sober. That's usually the issue. Nobody will take in a person who is high or drunk or violent. But the person can still eat there. Salvation Army is another great resource.