What to do when your friend replies in short answers?

He is one of my best friends, but he doesn't reply much, when he replies, it's usually one word like "yeah, sure, alright" I keep saying questions to him but still he doesn't try to say anything, and he is like my little brother but sometimes I just don't know how to continue a conversation with him, it's rare when we talk and he actually talks with me but still, as I said he is like my little brother, I don't wanna lose him, or him feeling uncomfortable for me not replying to him when I don't know what to tell him...
What to do when your friend replies in short answers?
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