I would say mal-uh-kye
How did you say it and how does he prefer it said?
I really don't like the name myself. It just sounds evil, with the "mal" beforehand (which means evil in a few languages), and the hard "k". Plus, I'm reminded of the lead evil kid in the movie Children of the Corn, which terrified me as a child.
Also, I think it stands out in a sibset combo with Aidan, Finn, and Calla. Those names are short and sweet and Malachai sounds like the red-headed step-child of the combo.