Jaslyn,
I din bring him to see a doctor, but i am sure he is slow to talk. Imagine, he can't even call Mummy. When he went to child care, he just cry and baby talk and it really makes me extremely worried because no one's child faces this situation.
I believe boys are slower than girls. Different child has different capacity and once their capacity arrive, they will perform themselves. I believe he is learning how to talk in his heart and he did not want to open his mouth to talk at this present moment. Why not you try to encourage him and talk, but don't talk baby talks to him, or else he will not speak our language.
I have 2 kids now. My gal learns to speak very fast because my auntie looks after her. She will talks to her and plays with her everyday without fail. Whereas my son is taken care by maid. She don't talked to him as much as my auntie and he is left alone almost the whole day, so less communication occurs.
Give him some time. He is fine and will be fine.