[1]s[on] o[f] [2]single

would mean that the entry has writing like " s o" " single"
transcribers are encouraged to record only what they see.
so the transcriber has interpreted this as " son of" and "single" showing the added letters with [ ]

the [1] and [2] probably infer note 1 and note 2

giving the overall feeling that he is a male child and his mother was Single !