son, my son
伜 centers on a male child in a familial context: a son, especially one's own son.
伜 combines the person radical 亻 with 卆, which likely contributes a phonetic element. The exact historical development of 卆 is uncertain, but the character is used to represent a son or male child.
A person 亻 beside 卆, a shape that resembles a small child with a hat. Imagine a parent proudly introducing their son: 'This is my little one.'
For サイ, imagine a proud father saying, 'This is my son, Sai.' The name Sai cues サイ and fits the meaning of a son.