There is this boy, I like him and he likes me too but I discoved that he still has some feelings for his ex girlfriend but he is telling me that he forgot her and every thing about her but he is lying I can tell.. what should I do ?

You most probably will get hurt if you continue dating him.
Avoid him until you are sure (you can never be sure anyway).
I had the same problem, and I broke up after 3.5 years...

Well you should probably first realize every one is going to have a little bit feelings for there ex's because they were once with them. He likes YOU now. You should stop being insecure and try not to ruin somehting good.. like this...