My son is 12 and 5'11.Hes always been tall and seemed to grow taller almost every month. But now he's been 5'11 for a while. Is he done growing?

Possibly. Individuals grow according to their genetics. How far into puberty has he gone? Beard? Muscular? Deep voice? Adam's apple? Adult phallus? Males potentially grow until 18-20 yo. But this is very variable according to family inheritance.