It was simpler for me when I thought about it being someone ELSE and not my kid. Somehow I still wanted to make excuses for my own child.
Adult choices mean adult consequences. It's really that simple. You can explain it to him very matter of fact... setting boundaries in order for him to live with you is totally acceptable.
My son had to challenge the boundaries and he had to leave for awhile. When he came back there were ~once again ~ clearly defined boundaries and consequences. He was able to live with me once we both understood I meant business.