There was actually a very interesting theory I read somewhere out there regarding Ven's likeness to Roxas and Terra's Xehanort. Someone suggested that Master Xehanort (the old guy) possesses Terra's body just as Xehanort's heartless possesses Riku's body at the end of Kingdom Hearts. Based on the way Xehanort's heartless talks about taking over Riku's body, he makes it seem as if it's something that he might have done before.
Along the same lines, he suggests that perhaps Kairi's heart wasn't the only extra one Sora stored in his body. He thought that maybe Ven's heart was somehow placed in Sora's body so when Sora released Kairi's heart and created a Nobody, the Nobody took Ven's heart and his form, thus creating Roxas, a Nobody with a heart (thus enabling him to wield a keyblade). There was a lot more to this which I cannot remember, involving some "evidence" which was only mildly convincing.
Still, given the manner in which the Kingdom Hearts plot has thus far progressed, I'm willing to bet that such a relationship between Sora and Ven is far more likely than a father/son deal.