Why does he do these things?

it seemed like he didn't care?

I can say for your fella but I can talk about what was gong through my head when I was your age. Your just starting out in the dating world and your developing and learning the dating skills and rules. This is difficult, hard and an emotional minefield at a time were your also very vulnerable to having your feelings hurt. I suspect its a little more complicated for blokes because your also discovering your masculinity at the same time. And being a modern day man is really complicated. Movies are no help, you watch 007 seduce a sexy woman by acting cool and cold and think oh so that's what women want; when really women want you to just be yourself minus all the stupid think you say and do with your mates.
So to summaries, when I was that age I was trying to work out both how I wanted behave when I was with a woman and how she wanted me to act.

does anyone have any ideas why he went all weird after that night?
When I was learning these skills its was a very vulnerable time of my life, so in order to protect my feeling I would go quite if I felt Id said something wrong or if I just paniced or thought about it for too long.

Also what does it mean when a guy looks at you blankly?
It means he is looking at you, but is hiding he's emotions. In this case he probably likes you but is terrified to admit it to you, for fear of rejection.

& any tips on how to talk to him?
Easiest way is to get him on his own. Don't try to talk to him with lots of boys around, that will put him under so much pressure.
I have said this before on this site and been heavily criticised, but that's not going to stop me from doing this again. It's 2008, if your a girl and you like a boy, then just ask them out. There is nothing wrong with a girl asking out a boy, I know America has some contrived dating rules, must of which seem to involve telephones and telephone numbers; but its still okay for a girl to ask out a boy.
If you do ask him out I recommend going to the movies. The first 2 hours your just sat together, this will put him at ease, then after you have something talk about.
If your not comfortable with the idea of, you asking him out (this could help you understand how awkward he feels) then do the other thing women do that is just make yourself blatantly available. Oh you want to see that movie, I want to see it too, I'm not doing anything this weekend?