https://www.urbandictionary.com/defi...?term=Sonshine
That's a great band and song by the way.
A quick google appears to suggest you can spell it either way. You'd definitely spell it sunshine in the "you are my sunshine... you make me happy when skies are grey" context but in a cockney geezer way of addressing a young man in a patronizing way, son shine might be more appropriate.
Old Frank from Cambridge is on wind up again I see