"His mom wants to study Dutch."
Translation:אמא שלו רוצָה ללמוד הולנדית.
It's not used so much, but not for the reason you give. For most nouns we would say ה... שלו, for example הטלפון שלו and it is quite correct. For some reason we treat family members, especially close relatives, grammatically different. I'm not sure why. You would usually not say האמא שלו, you could but it would sound a bit strange and possibly disrespectful.