I love the name
Silas! I'm not sure how it'd actually go on a kid, it seems like a bit of a heavy name for a five year old, but then again, I guess it's the kind of name you grow into instead of growing out of.
Sal works as a nickname, I guess, and Si [as mentioned by someone else] is still a better bet than
Cy.
Cy just doesn't make sense, and Si sounds just the same... besides, how often would any theoretical
Silas get his nickname actually written down? I know a lot of people who go by Si...uh, Simons for one, and it works.
As for middle names, I love the name
Oscar, but I guess
Silas Oscar has too many s sounds. Maybe it'd work with a surname which would balance it out?
Silas Morgan and
Silas Dorian are good, but I've never been entirely sure on whether
Dorian is -too- pretentious or -too-
Oscar Wilde or just a good name.