quesita?

If he calls me "mi quesito".. what is the translation?

I've heard of "the big cheese" before, but never "the little cheese"!

Queso is cheese. -ito is a masculine diminutive ending. So, quesito would be little cheese. Maybe it's being used as a term of endearment? Maybe it has a more specific meaning in some region?

Well, in Argentina, "queso" is colloquial term for a person who isn't very good at something. 'I suck at Maths' would be (colloquially), "Soy un queso en matemática".
Although, 'mi quesita' or 'mi quesito' does sound more like a term of endearment, but probably for something you did at that moment.

Ro