Angry with his two-year-old stepson for crying at night, a man slapped the toddler in June last year, leaving red marks on the child's face and neck.
The 29-year-old physically abused the boy on two more occasions that month, a court heard.
Yesterday, the man was jailed for six months and two weeks. He cannot be named because of a gag order to protect the boy's identity.
One evening in early June last year, the man and his wife, who is the boy's mother, were asleep in their Yishun flat when the child's cries woke them up.
The woman, 28, then heard her husband scolding her son, who had stopped crying. She wanted to turn on the lights but the man forbade her from doing so.
Using the light from her mobile phone, she noticed red marks on the boy. Her husband later admitted that he had lost his temper and slapped his stepson's face.
The toddler angered his stepfather again a few days later when he wet his nappy. The man then pulled the child's hair and shoved his head onto the floor.
The boy burst into tears, drawing his mother's attention. She found some bruises on the child's forehead and noticed that some of his hair had been pulled out.
The man abused the boy a third time later that month. When his stepson refused to sit still, the man pushed the boy and he fell.
The boy's mother later noticed redness around the toddler's nose and, on June 15 last year, alerted a social worker about the three incidents. The social worker took the mother and son to KK Women's and Children's Hospital, where staff alerted the police.
In an unrelated incident, the man went to Bukit Merah Polyclinic at 11am on April 28, 2016, as he had a headache. He got angry when a doctor gave him only a day's medical leave. He told the doctor: "If I see you outside, I will punch you."
Separately, the man was part of an unlawful assembly at the void deck of Block 233, Ang Mo Kio Avenue 3 at 2.30am on May 3 last year.
All the participants were from the same secret society. They had gathered for a "settlement talk" after one of them was accused of being disrespectful to the godmother of another member.
There was a heated argument, but a fight did not break out and the parties were leaving the scene when the police arrived.
The man pleaded guilty to one count each of ill-treating the boy, being a member of an unlawful assembly and using threatening words on a polyclinic doctor.
Two more counts of child abuse involving the same boy and one count of behaving in an abusive manner towards a second doctor were considered during sentencing.