User blog comment:GotMilk5101520/Silver Backstory & Tartarus and the Tower of Heaven/@comment-91.125.29.199-20140603172131/@comment-7418318-20140603172945

Natsu is quite famous throughout Fiore and even a bit beyond from what Jose said in the Phantom Lord arc and his fame has probably grown more since then. Particularly with his victory over the Sabertooth Twin Dragons, which people throughout the entire country could watch live on Lacrima Vision (the village that Gildarts visited looked pretty remote and even they could see it).

Or maybe Silver was somehow inloved with Igneel in the past and he knows that Natsu is Igneel's adoptive son.