I say Silver is Gio's son, for a few reasons...
1. The big one. Their personalities are very similar, including stealing other people's Pokies and basically treating them like servants (at least before Silver realizes the error of his ways, and Gio's Persian may be an exception.)
2. I think that Silver's true reason for hating the Rockets is because he dislikes his own dad, perhaps because he wasn't treated too kindly. Not too surprising on Gio's part.
3. If you look at their expressions, they aren't really that different. They both have a little sinister touch to them. Their hair is extremely different though. XD
4. They both want to be the toughest trainer in the world. And both seem to care about strong Pokies and victory over most things. (Again, Giovanni's Persian may be an exception)
BUT... Who's Silver's mommy??? XD