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You may want to sit down with him, alone, and softly ask him why he is so quiet. Hopefully he will give you an honest answer. If he is a little shy, it may take several sessions until he can trust you with everything he has to say. Sometimes people have dark secrets from an unpleasant memory. It sounds like he is interested in you, so I do not think he is losing interest. IF you really like him, try simply sitting with him and allowing the silence to be. You may find that, eventually, he will start the conversation. A guy like this may take a lot of patience, so, if you like him, give him time.