Kommentar zu Dewarim 2:33

ואת בנו [AND WE SMOTE HIM] AND HIS SONS — It is written בנו, "his son" (although read as בניו "his sons"), for he had a son who was as mighty as himself (Midrash Tanchuma, Chukat 25).

It (בנו) is written, etc. Since בנו lacks the letter yud, it [means “his son” and] implies that Sichon only had one son. Yet, since both syllables are vocalized with a kamatz, it [means “his sons” and] implies that he had many sons. Rather the word בנו indicates that Sichon had one son who was clearly recognizable as his son, for he was as mighty as his father. Therefore, it is written בנו (his son), without a yud. And this verse is informing us that he was killed just as all his other sons were killed.