As a child, I imagine I felt similar to someone who is a surgeon in his home country. He decides to move to America. But he does not speak the language yet. He gets whatever work he can to provide for his family. Day after day, people treat him like he is stupid as he sweeps and mops the floors of a school. He is there because this job allows him to be immersed in the language and listen in on classes where the language is being taught, while still providing the necessities of life for his family. But from the outside, he is dressed like a janitor and people assume that because an adult is doing work like that, he must have a very low IQ.
They treat this very educated and intelligent man as though he were stupid. They poke fun at him and talk down to him. He may not understand their words at first, but he is not foreign to human interactions. He can see their faces, hear the tone in their voices, look into their ridiculing eyes....he knows perfectly well what they think of him. And day after day he submits himself to this humiliation because he has a bigger picture in mind. He has a purpose. He is simply trying to learn the language first. Then, most certainly, one of those boys who mock him, will one day end up on his operating table and will be stunned to realize it is the janitor who is about to cut them open.
This is how it felt for me my entire life growing up. I felt like everyone was talking down to me instead of treating me like the adult I thought I was. I had the capability of understanding every thought process, it was just in a language that I couldn't speak. Does that make sense? I couldn't understand them because I spoke a different language, not because I was incapable of understanding.
Your child is learning a foreign language, but there are many things he comprehends. Treat him intelligently. I was speaking with a mother the other day who was concerned that her son may or may not quite know who she is. When there is a picture of her in front of him, he will point to it and acknowledge it is a picture of his mother. When she stands in front of him and asks, "Where's mommy?" He just looks at her - she thought his look was one of bewilderment. But when dad will ask "Where's mommy?" Their son points to mom. If you assume your son is intellectually challenged, this could be a concern. But if you assume he is intelligent but just hasn't learned how to express his genius in this language yet, it seems very obvious what is going on.
The look of bewilderment was a look of, "Really?! You're seriously asking me where you are?" It's a feeling of, "Please. Don't insult my intelligence. You're asking me where you are. That is the most ridiculous thing I've ever heard." When asking who someone is in a picture, well, maybe you don't recognize yourself. I can understand that. His body doesn't look the way he remembers looking either. It's small and strange looking....only his eyes reflect something close to what he remembers looking like. When dad asks him where mom is, he points to her. Dad may have just overlooked mom.
Parents wonder why the intellectual process seems to progress so slowly. His mind needs to be stimulated. If you repeat over and over again the same things and in partial sentences, then that is how he is learning the language and learning to express himself....repetitiously, and trying to fit a lot of meaning into a couple of words. No wonder it takes so long to find what he wants to say.
If someone is trying to learn a language and is only taught a few words, then he is not going to be capable of carrying on a full conversation. At best he may only be able to use a couple of words here and there with a lot of gestures and grunts sprinkled in. If you want your child to speak in complete sentences, then use complete sentences when communicating with him.
Also, speak slower, not louder. They are learning a new language, they are not deaf. In fact, if they are anything like the little boy I care for now, their hearing is excellent. He hears the most faint sound and must know what it is before life can move forward.
So. In a nut-shell. Use full sentences. Treat him like he is intellectually up to par or above his cognitive age, not his physical age. Speak slower, not louder. Give him time to learn a new trait before stepping full-board out there on his own. Expect him to progress neuro-typically according to his cognitive age and treat him that way. Then he will. :) Time-lines are different, but the progression is still the same. I cannot stress enough how important it is to treat him like an intelligent, bright, reasoning human being. He takes his queues from you - make sure they are the queues you want him to base his behavior on.