If he is self-employed, can't he show documents and papers of his company or work and what he does? If you haven't met yet, I'm not really sure if sponsoring him will satisfy the ECO that he's going back to the Philippines. It's really hard to convince the ECO when it comes to ties to the Philippines.
He can show proof of work, yes. It's the part about reasons to go back that I'm most worried about now I guess.