IF my dad was dignosed with demensha and altzimers in 2007,and my mother and he had a will drawn in 2003,and my mom passed in 2008,and he just passed in 2009,but changed the original will in nov 2008,could he legally do that?

If an Attorney and two witnesses thought he was capable, yes. If you don't believe he was then you will have to retain an Attorney, challenge the "new" Will and present your proof.

Depending on what the original Will said it does make sense for him to write a new Will following your mother's death. Whether he was capable of doing so is going to be the question.

Why is the question "rights of stepchildren" if you are asking about your mother and father?

You can challenge the will if you want, but you will have to get professions who treated him at that time to testify he was not in his right mind at that specific time.