The boy was kept in hospital overnight after the attack, which had aggravated an existing medical condition.
The incident happened on Sunday night, when the boy and his 10-year-old brother were walking along Brockburn Road in the Pollok area of Glasgow.
Just after 10pm, the pair, who are both Asian, were approached by a teenager who hit and racially abused the older boy.
The victim made his way home and was taken to hospital by his parents, Police Scotland said. He was released from the care of medics earlier today.
The suspect is described as being white, aged 15 to 16, of slim build and with short, light blond hair. He was wearing a dark body warmer, a red short-sleeved shirt underneath and grey jogging bottoms.
Police want to hear from anyone who saw the incident or who may have seen the boys or their attacker that day.
Inspector Fraser Lennox, of Pollok police office, said: "The boys had been at their aunt's house nearby and were making their way along the footpath when the assault took place.
"Officers are checking CCTV and carrying out door to door inquiries in the area in an effort to trace the youth responsible."
Meanwhile, a man was left fearing for his family's safety after being racially abused by a six-strong gang.
The 34-year-old, from Pakistan, was taunted by the group of around four males and two females when he went to a shop near his home in the Wester Hailes area of Edinburgh.
The "distressing" incident, in which they shouted racist insults at him, happened at around noon on Saturday in the area of Hailesland Grove.
Appealing for information, Inspector Stevie Dolan said: "This was a very distressing incident for the man, who said it left him fearing for the safety of his family. People should be allowed to nip to their local shop without being abused and intimidated by strangers and we are working hard to identify those responsible."
Police can be contacted on the non-emergency number 101.