Why would a guy get nervous when alone with a girl?

He used to like me a few months ago but then he got with my friend. Last week, me & my other friend were revising & he was overly complimenting me (even my friend noticed). He started looking deep into my eyes when I was talking to my friend, like he was trying to read me or something. Then it was just me & his friend & we were just talking about random stuff. I always catch him staring at me smiling for a really long time and even if I look away and look back, he's still staring at me. Earlier, we were messing around and he grabbed my hand and grasped it really tight. This is what he's always like when we're in school. Then I was walking home and a few of my friends decided to walk the other way, leaving just us together. It's the first time we've ever been alone together outside of school. He seemed really nervous and didn't always know what to say to me. But he made it obvious that he wanted me to stay with him. We were talking about what I'm like when I have a crush on someone and I was explaining I'd rather them be upfront and he was questioning me on it. We spoke about so many different things. He is normally quite a loud, outgoing person but he couldn't even look me in the eyes when we were talking (he always does, even when we're not talking). He just seemed really nervous and tense - was this because we were alone? The other thing is that I really want to text him so we can get closer (I got the number off a friend). I've been waiting for the perfect time and I think this may be it as we ended earlier on a really high note. Should I go for it and text him or am I moving too fast?
Why would a guy get nervous when alone with a girl?
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