This kinda well known manga but i forgot it .....

Story about a boy whose father was a lord in japan feudal era, his father is killed and the enemy took his mother as his wife.

The mother then promised to make her son a monk when he turned 18 because the enemy afraid the son gonna take revenge on him.

I think the son was sickly and get swapped with our mc but i kinda forgot but i remember the son then make a plan to take his father land back.

Probably Shanaou Yoshitsune?