Hugo… Aeroc’s butler saw him and cried because he was the splitting image of Aeroc. Rapiel jr thought he was some crazy old man, until he was told to check Rapiel Westport’s tombstone, seeing the discrepancy… it all clicked.
He remembers Aeroc… not too sure because Rapiel jr story is not accessible to online readers, it’s a special epilogue available on printed books. I read the spoilers on a blog, it was hard to understand…
The rest of the information.. he got it from Wolflake (his the guy with the grey eyes in chap 31) in exchange for a sexual relationship. Not sure who initiated though…

Who told rapiel about the truth of his birth and aeroc's death