The good news is that most developmental charts put independent sitting at around 9 months, so at 8 months your son is still well within the normal window, no need to panic just yet. The fact that he's holding his head up steadily and rolling around actually shows his core and neck muscles are developing, which are the building blocks for sitting, so those are really positive signs. On the other hand, every baby moves at their own pace, and if by 9 months he still isn't sitting unsupported, that would be a more meaningful signal to flag. It's worth doing a quick check with your PD at his next developmental review just to get a professional eye on it, they can assess whether some targeted tummy time or gentle exercises might help move things along.