also: おとむすこ
younger son
Refers specifically to a younger son, as opposed to an older son or a younger brother. Not a common everyday word; more often seen in literary or explanatory contexts.
彼は三人兄弟の弟息子だ。
He is the younger son among three brothers.
弟息子はまだ学生で、兄はもう働いている。
The younger son is still a student, while the older brother is already working.
息子 simply means 'son', without specifying birth order.
長男 is the 'eldest son', the opposite of 弟息子 in birth order.
Standard kanji spelling.
Contracted reading; less common but attested.
Compound of 弟 (younger brother) and 息子 (son). The reading おとむすこ is a contraction of おとうとむすこ.