Let me ask you a question: Did the prodigal son's father know whether or not his son had changed his way of living when he welcomed him home?
Apparently not. The Bible simply says that "while he was still a long way off, his father saw him and was filled with compassion for him; he ran to his son, threw his arms around him and kissed him" (Luke 15:20). This son had wasted his inheritance and turned his back on his family–but the father still loved him, and joyfully welcomed him back. If he had refused to do so, the son might have returned to his old ways.
I don't know your whole situation, of course, but this is what I hope you will do with your son. It has taken courage for him to swallow his pride and make contact with you; don't blow out the flame that is flickering in his soul! Even if he hasn't changed completely, something good is going on inside of him, and he needs your love and encouragement, not your condemnation.
This, after all, is how God deals with us. We aren't perfect; we have sinned, and we aren't worthy of being called God's children. But God still loves us, and He welcomes us home when we turn in faith to Christ. He doesn't wait until we're perfect; He receives us just as we are.