What if he continues to be shy?
Then he probably isn't so comfortable around her. He probably likes her but he's not sure if she likes him. She/you (I guess) should probably just ask him something like in school or stuff like that. If you're in school together. My crush asked me what cake we should bake (the group we where in) and that turned into an good conversation. It was on Snapchat.
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What would be the circumstances for him to no longer be shy due to losing interest?
That would probably only happen in the following cases1. She is now dating another guy, so he tries to move on from liking her.2. He found out something negative about her that ruled her out as a future girlfriend, like she slept with guys he knows, did drugs or something like that.
How about if the guy crushes on another girl?
If he had a proper crush on girl 1, he won't genuinely like another girl for a while after. But it is possible if he meets another girl of similar or better attractiveness, that he might then move on and stop crushing on girl 1, but usually this would be after a few months chasing girl 1 to no avail.