Your son needs to be assessed towards his developmental milestones, especially gross motor as your concern is towards him still not sitting unsupported at 9 months age (which is definitely late)
If the delay is only in gross motor milestones, (as against a global delay), then specific workup needs to be done depending on your examination findings!
Next Steps
Surprising that you didn't raise these concerns with your doctor at time of 6 & 9 months vaccination!
Or are you just taking a 2nd opinion online?
Health Tips
In certain issues like your case, detailed history & examination has to be done: no shortcuts (or worse, soothsaying)