Where we live has rules outside of our control too and it feels crappy that we just can't follow what our heart says for my son. He has to be full time in school or he simply can't live with us. I know the guilt of not having the money to truly help in the way you feel like you need too. Knowing your limits will help you not feel as bad. Next time he needs food take him to a food bank. And it he complains tell him your on a limited income and you can't help him with money. What you can help him with is to find resources.

He's only 21 so job corp may take him for free. College as a homeless youth may take him in special programs they have dedicated to homeless youth. that would require student loans though. I always go the route of education when homeless because that gives them a room, food and resources with the string attached that they have to show up and try in classes.