I hope you can clear the doubt that I have been having regarding setting off proper nouns by commas. In the following example, taken from your Grammar Book, you indicated that Andrew was considered to be the only son because his name was set off by commas. I know you are right, but can you please give me the rationale for this assumption and refer me to any particular grammatical rule that may be applied to this usage. 

My son, Andrew, is a law student.
My son, whose name is Andrew, is a law student.
I have one son and his name is Andrew.

The comma is a highly logical marker. When it demarcates sequential nounal elements, or fails to demarcate them, it is doing so for a meaning-making purpose. In this sentence:

 My son Andrew is a law student,

the two nounal elements `my son' and `Andrew' are not demarcated, which leaves them in apposition, such that they act upon one another as noun and adjective: `Andrew' effectively describes `my son', or vice versa. This adjectival capacity of each noun prevents either from functioning as an undistributed naming element. So the logical template of this sentence is: `Some things that are `my son' are some things that are `Andrew'. Since some things that are `my son' are some things that are `Andrew', and vice versa, Andrew is one of my sons, not the only one.

However, when the comma breaks the apposition, the nounal elements cease to impact upon each other as a noun-adjective combination. Rather,  `my son' and `Andrew' are alternative names for the one entity:

 My son, Andrew, is a law student.

The `one entity' logic of this naming sequence entails that `my son' is the undistributed noun-subject, such that the logical template of this sentence is: `All things that are `my son' are some things that are `Andrew'. (Clearly, `Andrew' cannot be undistributed: it is not an exclusive name.)

Now, since all things that are `my son' are  `Andrew' , I have no son but Andrew.

(This point is perfectly clear in propositional logic. Theory of syntax has been rather tardy in picking it up, despite the fact that comma usage is so highly dependent on the distributed/undistributed distinction.)

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