Who was in your opinion the closest man to Dragon, Kuma or Ivankov?
In my opinion it was Kuma because he knew that Luffy was Dragon's son, while Ivankov was unaware of the fact that Dragon even had a son.
Ivankov had full access to newspapers when he was in Impel Down.
He was also present during Sabo's rescue 12 years ago, when Luffy was 7
It all depends, because Kuma has a built in cheat mode system that allows for him to know specific details about wanted individuals, mainly their full names.
The World didn't know Dragons full name until Sengoku announced it during Marineford. Kuma would be able to know his full name simply by looking at him. From there it wouldn't take much effort to deduce that him, Garp, and Luffy are all related.
If Dragon did tell Kuma, then you also have to factor in, when did Kuma learn this information? Was it after Luffy became a Pirate? Was it like a "Hey Kuma, my son is now a pirate, so if you run into him on the sea, make sure you look out for him" type of deal? If so, Ivankov was in jail at that point, so he wouldn't have been given the same information, cause he didn't have contact with Dragon at the time, and if he did, what good would it have done with him being in jail.