I feel like telling a homeless man to go crawl in a trash can
Why tell him to do something so degrading?
To be honest, I'm not sure.
Just because he's homeless doesn't give you the right to treat him that way. Instead, you should be trying to help him get back on his feet.
I think I can prove that he's not worth the effort.
That's a pretty awful thing to say about another person. Trying to help anyone in need is always worth the effort. Think of how rewarding it would feel.