The boy was out throwing snowballs with two of his friends in the courtyard of an apartment block on Sunday evening when the incident occurred.
When one of the snowballs crashed against his window, the 43-year-old man lost his temper and walked out into the yard.
“He was very irritated when he came out. He grabbed the 9-year-old and punched him in the face. Then he kicked the boy when he was lying on the ground,” police spokesman Tord Carlsson told Aftonbladet.
The boy’s friends, a boy and a girl aged 10 and 15, tried to intervene. They too were struck by the older man, who has been detained in police custody on suspicion of serious assault, Aftonbladet reports.