I'm confused by Metlieb's comment (or maybe you edited yours?)...
Anyway, "Umie twój syn liczyć" isn't totally wrong, but it's strange. It really doesn't sound natural to put the subject after the verb in Polish. I believe the only sitation when it is (perfectly) fine is when you use Formal You forms. "Umie pan liczyć?" is a normal, natural sentence. Well, putting aside whether asking an adult stranger if he knows how to count is natural.
Wouldn't "moj umie" be better translated as "mine knows how" to rather than "mine can?"
So what is wrong with 'Is your son able to count?..' There is nothing lost in the translation and umieć is 'to be able...to do something'.
Dlaczego jak dałem "my can" mi nie zaliczyło? Jaka jest różnica w takim razie między "mine" a "my"?