grandson
Rare term; the usual word for 'grandson' is simply 孫 (まご). This compound explicitly specifies a male grandchild.
The expression 孫息子 is rarely used; normally 孫 is sufficient.
孫息子 means grandson, the male counterpart to 孫娘 (granddaughter).
Compound of 孫 (まご, 'grandchild') and 息子 (むすこ, 'son'). The exact historical derivation is uncertain, but it follows a pattern of specifying gender in kinship terms.