Stout is a sub-category of beer. Beer includes ales and lagers, depending on the brewing process, which can be broken down further into groups like stouts, porters, sours, pilsners, IPAs, etc.
Like how bourbon and scotch are both whiskeys, and whiskey, vodka, tequila and gin are all spirits.
If you wondered 🍻
Anyway, yes, he can buy you a Guinness.