I speak of my son

[Marina Gaidar]

What is the difference between "I speak of my son" and "I speak about my son"?

[nelson13]

It can be the same.

In the OALD, for SPEAK, sense 3 doesn't make a distinction between them:

speak of/about something/somebody: to mention or describe something/somebody

[Tdol]

It can depend on the amount of information- sometimes of is used for a brief piece and about for a more detailed description. I would not use speak of if the central subject of my talk was my son.