My 2 1/2 year old son wont use the potty!?

My daughter isn't there yet, but my best friend's son wouldn't use the potty when he first started at 2 1/2. You could ask him, do you know what the potty is for? "yep" do you know when you need to use the potty? "yep" do you want to use the potty and not diapers? "nope". He knew what to do, he just didn't want to. She tried potty books with elmo (his favorite), rewards, and underwear and nothing worked. So she decided to just wait a while. About 4 months later he just started using his potty all by himself whenever he wanted. Sometimes the best thing you can do is just wait.