I don't think it is true, rumours like this happen all the time. Besides, like one of the comments on the site said, I think Ian and Nina are professional enough to keep their personal lives aside, if that is the reason the site is claiming.
Plus they all have contracts for at least 6 seasons, since that is how long the writers originally planned to have so I don't think Ian's leaving.
And he was most probably joking when he said he would leave if Enzo was killed.
There are other rumours saying that he'll quit because it's awkward with Nina, and that she's involved with Michael Trevino. What? I honestly have no idea where they got this from, but according to sources it was a rumour that was fabricated, like other rumours usually are.
So this is probably just another one of those rumours. I mean most of the time a celebrity says or does something, even as a joke, it gets twisted into all sorts of rumours, so for now, I wouldn't worry about Ian leaving the show or anything.