A 22-year-old man is behind bars for pushing his girlfriend's son into traffic on Highway 121 in Grapevine, police say. Witnesses told investigators that Michael Jerome Walker also fiercely shook the 5-year-old boy and held him over his head on the side of the busy road. The child, his mother and Walker were all in a car headed to Fort Worth from Lewisville when the two adults began to argue around 11 p.m. Saturday. Police say Walker began assaulting his girlfriend, and she became so frightened she stopped her car in the middle of traffic and got out. Witnesses say Walker grabbed the boy and held him in the air on the side of the highway. The child managed to escape his grip, but Walker pushed the youngster into traffic. Motorists slowed down to avoid hitting the little boy. Walker is being held on child endangerment charges and assault, as well as on several warrants from other cities.