Why didn't Neil tell his father what he told Mr. Keating about his feelings regarding being an actor? Would the ending of the movie have been different if Nail would have discussed his feelings with his father ?

I think Neil felt trapped. He expected his father to be angry and say no. If Neil were assertive enough, he would have had quite the blowout with his father. This would have been a good thing. Neil would finally have had a chance to purge those repressed emotions he needed to convey to his father. Although he likely still would have been forbidden to act in the play, Neil might have had other emotional epiphanies that would have changed his decision to kill himself.