language obstacle or character?

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My son is almost 4 years old. Recently we found he has some probmems at kinder, includes: Don't talk to others. Don't play with others. Don't tend to response other's question. We analysed and it may caused by some possible reasons: 1. Language. My family speaking Chinese so my son's English is limited. 2. Confident. We think about this because sometimes the teacher just asking him some simple questions and we also ask him like this at home in English. He can response at home but not at kinder. And if we are accompany with him he will be better. 3. Character. He is a shy boy. Sometimes other boy just say "stop" louder, and he will fell afraid. I don't know is it because the limitation of language. We just moved to Melbourne recently and still in the process of settlement, so we can't afford another child. We know to have another child is better for both of them but we can't do that. So what I can think of is as, 1. Try to find other children to play with him. It's not easy because we don't have many friends here. Also we can't just ask his classmates to play with us just because we need it, it's selfish. 2. Language training. There are some of the medical centres for this but we don't which kind of centres is good for him. Is there anything else we can do for him? Any advices and comments will be really helpful. Thank you very much.