Every baby is different =)
My 8 month old started rolling at almost 5 months, crawling at 7 months, and now is trying to walk (I keep telling him to calm down, but he doesn't listen!). My sisters baby is almost 6 months and doesn't roll, barely can sit, and really has no interest in doing either. I am positive, though, that her baby will eventually do it. I would be concerned if he isn't rolling at 10 months or a year, but you should be fine now! If he corrects himself, maybe he just doesn't like the unbalanced feeling?