I think you need to start by understanding yourself better. Only then can you help your son.
From what you say, it sounds like you and your son are both introverts. This is not a flaw, it is simply a different way of being, like being left handed instead of right handed. However this can be difficult because society generally values extroversion more than introversion. School in particular. You need to watch out for signs that your son is being bullied, because the lone kid standing on one side is an obvious target.
Trying to make an introvert into an extrovert is a bad idea. It embeds the idea that being an introvert is "wrong" somehow, and forces the introverted person to go against their nature all the time. It sounds like this is what has happened to you, and you need to understand and accept this. Then you can avoid repeating the pattern with your son. You also need to get your wife on-side with this. Help her to understand how you feel and think, and then she will be better able to help your son.
However there is value in learning to cope with social situations. Treat social situations as practical problems which can be solved by learning how to negotiate them. Also, see if your son can spot any other kids who are loners, and see if he can make friends with them.
You say your son "cannot speak". I'm not sure from what you say exactly what you mean. Do you mean he can't speak the local language even though he understands it? Could that be because he doesn't feel confident? No doubt he has a foreign accent, and maybe he even sounds babyish to local ears because he is so unpracticed at speaking. That can make him a target of mockery and exclusion at school, which will make him reluctant to say anything. If this is the only problem then perhaps some elocution lessons will help him to blend in better. If the cause is something deeper then I think you need professional help. Selective mutism is a recognised condition, and treatment is available. In the meantime not practising speaking means he isn't improving, and that is only going to make the problem worse.
Talk to his school teachers and find out what is happening there. I have little doubt that his experience of school is not a good one, and that is doubtless making him unhappy. Also try to get him to talk to you about his life and his feelings. Talk about yourself as well, and help him to see how you cope with life. If he can see himself in you then that will help his self confidence, and also help him to trust you with his problems.
The above is pretty general and vague because its hard to tell more from what you say. Perhaps after you talk to your son and his teachers you will have a clearer idea of what the problems are and you can ask about them in further questions.
