LYNN – A man was arraigned in district court Tuesday for allegedly punching his girlfriend’s 9-year-old son in the stomach after the boy got upset because he saw the man having sex with his mother.Smiling in his mugshot, Phillip Morris Jr., 31, of 118 Beach St., Malden, was arrested Monday on a warrant for assault and battery on a child causing injury. He also had two default warrants on larceny charges.Lynn police said that on Jan. 24 officers were dispatched to the boy’s Kernwood Terrace home where his biological father was having an argument with the boy’s mother. A short time earlier the boy told his father that Morris had punched him in the stomach the day before, police said.According to a police report, the boy walked in on Morris and his mother having sex in her bedroom.The boy got very upset about what he saw and the fact that his parents recently split up. At some point because of the way the boy was behaving Morris allegedly punched the third grader in the stomach with a closed fist.After the boy told his father, the man confronted the mother about what happened when he returned the boy from a weekend visit. The argument between the two escalated and police were called.The mother admitted that Morris had punched the boy. She also said the boy kicked Morris in the groin and hit him in the face after being punched. Police also interviewed the boy who reiterated a similar story.As the parents were being interviewed by police, both the man and woman alleged they had been assaulted by the other party. Each will be summonsed to court for domestic assault and battery.Police filed a report with the Department of Children and Families on the incident.Judge Stacey Fortes-White ordered Morris held on $500 bail at his arraignment Tuesday. He is expected back in court Feb. 24 for a pre-trial hearing.