Anything is possible actually.
1. Silver is Gray's dad/brother
That will explain about why Natsu said that he smell like Gray. But, then why Natsu never find out that Cana and Gildarts are related by blood? And I have no idea why Gray's dad/brother hates him.
2. Silver is Gray's future self
That will also explain about that similar smell. Maybe Gray made some mistakes and hate himself, changed his name, join Tartaros, and turned into a demon?
3.Silver is Ul's husband (and Ultear's dad)
That will explain why he hates Gray, because both mother and daughter sacrifice themselves for Gray, maybe? There was also a scene in chapter 353 when he visited someone's grave. Maybe that grave is Ul's (made by himself or something)? But about the similar smell, I have no idea.
4. Silver is Gray's dad/brother and turned into Deliora
Someone in the forum have that theory. Well, Mashima-sensei never reveal Silver's demon form and it is possible that his demon form is Deliora.
5. Or maybe both? Silver is Gray's dad and Ul's husband so Gray is actually Ul's son and that would make him Ultear's brother LOL (that was really messed up).

Well, who knows? Let Mashima-sensei reveal about that someday.

To answer you :
To your 1) I think he couldn't smell that Cana was Gildarts' daughter because they never lived together, Gildarts was unaware he had a child, so they couldn't have the same smell I guess
2) I saw this theory on tumblr too, but I think it's unlikely, but it can be that
3) Also saw this on tumblr, but Silver left Ul, he abandoned her, so I guess that means he didn't want to be with her anymore, he had a reason to leave her, don't know ! But that would not explain why he smells like Gray
4) I'm not sure, Deliora died, he turned into a giant stone and broke, if I remember, and that would mean Silver is from Zelef's book, because Deliora was a demon from his book
5) it can be true, yeah

Can't wait to see their meeting *w*

^ About Cana and Gildarts, yes, they never live together. But Natsu already know Cana since he was a kid as well as Gildarts. He is a good friend of Gildarts. Then why Natsu didn't recognize at all that Gildarts is Cana's father? And I highly doubt Gray and Silver ever met each other before. So how can Natsu immediately smell Gray on Silver? Based on Cana-Gildarts thing, I doubt the theory that Silver is Gray's dad. (Except Mashima-sensei forget about the smell in Cana-Gildarts case or you can say a plot hole).
If Natsu smell Gray on Silver, then the biggest possibility is Silver is Gray's from future/dark side of Gray.
If Silver turn out to be Gray's dad, then Mashima-sensei really made a plot hole with Cana and Gildarts thing...

I really don't think that Silver is Gray's father. Because it was said that his parents died when Deliora attacked their city. ._____.
Although there is a chance where his Dad survived........Maybe it's Gray's cousin/uncle????? Idk....