A child is 50% Turkish and 50% Dutch living in the Netherlands. Which language should be spoken with him at home?
If the situation allows it, I would recommend offering him both languages. For never again in your life will you learn languages as easily as during your childhood. From a social point of view, too, this would be an enrichment as the child will be able to communicate with more people. Moreover, multilingualism may speed up his cognitive development. In order to become properly bilingual, sufficient exposure to both languages is required. You apparently need to be exposed to a specific language for a minimum of 25% of the total exposure to languages in order to master the language. Note that the quality of this input needs to be good. If both parents speak Turkish well, Turkish could be the language spoken at home. Outside of the home, the child will be exposed to enough Dutch in order to become bilingual.
If the Dutch parent does not master the Turkish language, he or she should refrain from trying to converse in Turkish with the child. And if the Turkish parent speaks Dutch inadequately, he or she should keep speaking Turkish to the child. Namely, the child will not benefit from being exposed to an incomplete, qualitatively poor version of the specific language.