Colorado has a flat 4.55% income tax. Deductions are mostly parallel with federal.
Denver has a flat "Occupational Privilege Tax" of $9.75/month for the privilege of performing work inside the city.
Don't worry about Denver County: there is no such separate entity. It's a unified city and county government.
Denver also has retarded property taxes, but those would only (directly) affect if he owns property in the city, which is a shitty idea on so many levels. By contrast, I pay $2200/year on $400,000 assessed value in the suburbs.
Sales taxes are high-ish: Denver might be above 10% now and most of the suburbs are better but not by much.
Depending on where in Meatchicken you guys are, I'd think about staying. I'm this >< close to moving to be closer to family in Holland-Muskegon-GR, and that's after I specifically moved here to "move to a free state. (Back in 1998 when it was, anyway. )