Hola George

If you are specifically asking about the boss as in "what s/he is like (to work for)" then you need:

because what you really want to know is what sort of personality / behaviour he/she normally has ( = what they are like)

A possible answer to this could be: 

"La jefa es muy simpática, así que te gustará trabajar con ella."

The boss is very friendly, so you'll enjoy working with her.

"¡El jefe es un ogro! Siempre está exigiendo cosas y tiene muy mal carácter."

The boss is an ogre! He is always demanding things and has a very bad character.

If you are specifically asking about the boss as in "what the boss is like that day" then you need:

because what you want to know his their mood at the moment.

A possible answer could be:

"Pues hoy está regular, un poco gruñón, no sé por qué."

Well, today he is so so, a bit grumpy, I don't know why.

"Pues hoy está de buen humor y nos ha traído donuts a todos para desayunar."

Well, today he's in a very good mood and he brought us all donuts for breakfast.

It helps if you think that in "¿Cómo es...? using ser, you want some descriptive essential characteristics (es simpático, es alto, es bueno, es un ogro...); and you think that with ¿Cómo está...? using estar, you want adjectives and phrases that express how this person is feeling ( estar for feelings: está enfadada, está de mal humor, está contento, está preocupada...)

I hope these examples clarify it a bit more.

Saludos

Hola George

For any general question where you use "how...?" in English we use "¿cómo...?  and if what you need to ask in English is "what something is like.." expecting a "description of something or someone" you still use ¿cómo es...? in Spanish. 

The common mistake by English speakers is to use "qué..." when they want to ask "what is ... like?" because as you have a "what" in your sentence, you may think you need a "qué.." in the Spanish sentence, but that's wrong as you still need a "cómo".

I think that's what you're asking but not sure...

Saludos