If there is already a crowd gathered around him, and you have no particular knowledge or equipment that is needed, then there is no reason to stop. It's already being handled.

Otherwise, refer to the Check/Call/Care.
1) Check if the reason he is lying in the street is hazardous to you. By this I mean a downed power line, the victim's mental problems, etc. Adding another victim that the EMTs will have to scrape off the pavement will not help matters any. If you're not sure, call first, then check.

2) Call 911, or whatever your local emergency number is. Get the professionals, with all the expensive equipment, on scene as fast as possible.

3) Care for them as best you can. This is where the legalese comes in. "Good Samaritan" laws protect you for anything you're trained to do. A basic first aid/CPR/AED certification allows you to treat shock, seizures, heart attacks, etc. It doesn't cover emergency tracheanomies with a Papermate and a pen knife. If you're legitimate first aid skills are too limited for the situation, go direct traffic so the emergency services don't get stuck in a traffic jam.