Most Helpful Girl
If you aren't sure if you like him, you probably don't.
It is okay for him to ask someone else if you agreed to only go as a friend date. I was someone's friend date and a girl he liked ended up wanting to go with him, so he went with her instead. The point of a friend date is that you go with someone casually, but most want to go to prom with someone who they like/know has interest in them, which you established that you didn't.
However if it bothers you, all you can do is let him know.