Teaching conversational english to toddler

My son is two and a half years old. Ever since he was born we have only been talking to him in our mother tongue and not English ( simply because we use mother tongue at home) . He can talk so well in that language. Can even narrate entire stories. With English he can understand individual words but when we converse with him in English he is unable to understand. We enrolled him in swimming and music class but he is unable to follow what the instructor says in English. So i have to be near and translate for him. How can i help my son understand conversational English. Can you suggest activities or classes to help him learn.