If possible, ask more specific, open-ended questions. For example, instead of saying, "How was your day?" try asking, "What was the best thing that happened to you today?" Maybe changing up the kinds of questions you ask and how you ask them will help.
You could also try having more conversations with him about things he likes. If he likes a certain cartoon show, ask him who the characters are in it, who the bad guys and good guys are. After you get him talking, try asking other questions and see if he opens up a bit more.
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