This guy that is normally hilarious and outgoing acts so shy and nervous around me!?

I would guess it's because now he may be in a place of confusion which would probably make him even more nervous than if he knew you didn't like him. It's the uncertainty and lack of closure. He probably has a lot running through his mind around you and is also trying to observe and feel things out extra hard now that he may be confused. If a girl rejects me but then tells me she likes me later, I always question if she meant it when she said it later, the contrast between the "like" and "not like" aspects with her and that stuff always lingers in my mind and I get uneasy as well as frustrated. He also may think your trying to play games with him(I usually do)and when that's the case, the more he wants to talk to you the more he will also be conflicted about actually entertaining that because he does not want to play into games(this is all being said if he is like me when it comes to all this.)

My suggestion is you find a way to make it more clear to him with out making him feel pressured about it. At this point, after jerking him around the ball is kinda in your court I'd say. You can't blame him for not knowing what to think so you have to show him what to think... Also, don't bother to put that message, that you like him, forth unless you mean it and feel you stable in it and if that's the case find ways to convey that to him also(that you wont play games/your for real)...