As we know, the older son of Duncan has fled to England to seek asylum under its king. Scotland and England were not allies at the time, but Duncan was related to the King of England. Thus, Malcolm was welcomed.
While there we can assume the physical safety of the boy, who seems to be played in film as much older than he would have been in real life, but his mental stability seems to be suspect. Per the fashion of the time, he knows he should gather an army to avenge his father's death, but he fears that his own youth may be in the way. The King helps him here by promising him the noble warrior Siward, his son, and an entire army to battle for his rightful crown. Later with the arrival of Macduff to encourage him, Malcolm is read to attack.
Thank you very much for your answer!!
I truly appreciate it.