@bigz007: Yeah me neither. Being his father seems like the most likely explanation but you never know with Fairy Tail.
@bigz007: Pretty sure it hasn't been confirmed, just heavily hinted at. There's a chance Silver is an older brother or something else, but he definitely looks the right age to be Gray's father.
...which would make Gray half-demon, I guess? That could be interesting. Though I remember a character saying Silver is in human form right now, so maybe conceiving a child in that form would make it human too.