Aki Tachinu Autumn Has Already Started (1960)

Single mum (father of child dead) from the country with 6th grade boy comes to Tokyo, leaves the boy to live with his uncle's family - wife, male, female teens, run a struggling greengrocer - and lives/ works in local inn. He gets bullied/ fights with local boys, goes to the baths, plays basketball, is befriended by his male cousin and becomes friendly with grade 4 girl who's the daughter of the innkeeper, who employs his mum and is the longterm mistress of a businessman from another town, who's the girl's father and who brings her half sister and brother to meet her but they're mean to her.