I agree with what has been said. His main problem has been lack of confidence and his grandmum hasn't been much help--telling him he isn't as good as his parents. In this book, one thing that has motivated Neville to do better, giving him more confidence is the return of Voldmort and his followers. It looks like it has given him a purpose in life and he wants to go after the people who hurt his parents. I expect more good things from Neville in the last two books. I wouldn't be surprised if he joins our trio in more adventures.