When he was a boy Franklin who afterward became a distinguished statesman and philosopher learned his trade in the printing office of his brother who published a paper in Boston.

The additional information in the sentence would, of course, feature with commas - 'who afterward became a distinguished statesman and philosopher'. 'Franklin', as the name, would feature within commas. More information is given about the brother too, so there would be a comma after 'brother' as well.

Alternate: A comma is placed after "boy" to separate the introductory phrase.

"Franklin" is set off by commas because the phrase "who afterward became a distinguished statesman and philosopher" is non-essential (extra information).

A comma is placed after "philosopher" to close the non-essential clause.

A comma before "who published a paper in Boston" because it's a non-restrictive clause (additional information about his brother).