I have 3 pieces of advice:
1) you could slowly start to distance yourself away from him (don't go out, be a slow reply etc.)
2) you could nicely tell him that you feel this way, and that you'd rather just be friends
3) wait a few days until your really sure you have no more feeling left before doing something about it.
After all your not sure if he likes you back yet he might just wanna be friends.
Maybe going out to the cinema made you realise you didn't like him as much..
Goodluck to you :)