Janski, I'd be glad if you could explain this.
Janski, I'd be glad if you
Could you explain this?
Justin Trudeau, the son of Pierre Trudeau,…
This apposition aids the reader in recognizing Justin Trudeau by supplying a more informative context than the denotation Justin Trudeau
alone. Make note of the grammatical (and cognitive) status of Justin Trudeau and of the apposition: both are at the same level and definite.
… a son of Pierre Trudeau
would serve, perhaps inadvertently(?), an additional function: it attributes to Justin the property of being P.T's son: being someone's son has its (dis)advantages and may have its consequences. The apposition is indefinite: Compare with The tiger is a wild animal.
In Justin Trudeau, son of Pierre Trudeau
, the apposition would also be attributive, but it would attribute to Justin his "son-hood" only.
… a son of Pierre Trudeau's,
the former… has a partitive reading (=one of P.T's sons)
…the son of Pierre Trudeau's, the former…
To make the last one grammatical, one would have to give it another restrictive modification needed to justify the fronting "the": Justin Trudeau, the son of Pierre Trudeau's that I know best,..