What does it mean when a guy waves at a girl but doesn't smile?

For example, my crush used to smile and wave at me all the time because he's a quiet guy. Then I just simply asked him to hang out one day and now he seems shy. When I see him, he will wave sometimes but doesn't smile like he used to. He just kinda puts his lips together to make a kissy face, and that's it.

(Note: he smiles and waves to my guy friends, even when I'm with them and then he ignores me but blushes and looks away if we make eye contact)

What does that mean?
What does it mean when a guy waves at a girl but doesn't smile?
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