You are probably right that his refusal is a form of defiance. Justin is at an age where defying the adults around him is the name of the game. If he perceives that you very much want him to say his name, then by refusing to do so he is exerting a small amount of control.
He is young to be saying his name. It probably will happen without any great effort on your part. But you can encourage it if you do so without too much pressure. Point to different people in a family photo album and ask, “Who is that?” If, when you get to his picture, he refuses to offer a name, say without undue emphasis, “That is Justin, isn’t it? That’s you.”
After you do this for a while, give him the album and ask him to point to different people. Supply the names as he points or calls on you. If he points to a picture of himself, pretend you don’t know who it is and ask him to tell you. You might be surprised at what he will say. You can think of similar activities to encourage him. The important thing is to keep it light.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.