My second conclusion is that Cid is Joe's son, with his Asian wife, who's name I seem to have forgotten. I have a few reasons for this:
1 - Cid appears to be of mixed race, he has tanned skin, somewhat Asian eyes, but also has caucasian characteristics.
2 - In a scene where Old Joe is remembering his Wife, you can see them smile, and the faint cries of a child.
3 - More may surface.
I love crazy theories as much as the next girl, but I think this one is a little too far off the deep end.
1. Neither Joe or his wife had tanned skin.
2. I think you were imagining those "faint cries." In the diner, Old Joe says "For a long time she thought we'd have a baby." That seems to imply pretty clearly that they did not.
But above all, how would their child get sent back 30 years into the past? And why would Sara say Cid was her son?
I'm way too lazy to ever be truly belligerently insane.