Just re-read chapter 793 again looking for a possible bounty increase rumour I heard about when I came across something I had overlooked before.
At the top of page 11 re-read the top panels with Eustass, he states that his "Old Man" was killed by Doflamingo and it was an "old" story, now I have a theory that he is actually the son of Corazon and Doflamingo's nephew.
If you don't believe me then take another look at his picture and try and not to see the family resemblance, more so to Corazon than to Doflamingo.