How would you respond to a smiley face text?

This guy and I have been seeing each other for a while and we are beginning to get kind of serious. But..he left for the whole summer for a study program on the west coast. So we basically only communicate by continuously texting whenever we can get to it. We were having a nice conversation when all he answered was (:

How on Earth am I supposed to continue a conversation for another 30 something days? The only thing I can think of is to awkwardly change the conversation.
How would you respond to a smiley face text?
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