In 'Macbeth', King Duncan's son Malcolm becomes king at the end of the play. As Duncan's elder son, Malcolm is the rightful heir and ultimately restores the Scottish throne after Macbeth's defeat.
Duncan names Malcolm Prince of Cumberland in Act 1, identifying him as the successor Macbeth must displace. After Duncan is murdered, Malcolm flees to England while his younger brother Donalbain escapes to Ireland. Their flight makes them look suspicious, allowing Macbeth to seize the crown.
Malcolm later returns with an army supported by English forces and Scottish nobles. He orders his soldiers to carry branches from Birnam Wood as camouflage, fulfilling one of the witches' prophecies that seems impossible to Macbeth. Macduff kills Macbeth, and then hails Malcolm as king. Malcolm's final speech grants new titles and announces his coronation.
Fleance is not Duncan's son; he is Banquo's son. Donalbain is Malcolm's younger brother and does not become king in the play.