Sitting with support is usually achieved at 6month of age and Sitting without support achieved at 8-9 month of age. This also can be slight early or delay in some babies. As rest of the milestones are achieved, he will attain this also. Start complementary feed now and continue
vitamin d till 1 year of age. Usually breast milk has sufficient
calcium to meet daily needs, no need of extra supplementation. Anytime a detail examination is better so if possible get a full Consultation.
Next Steps
just observe for some more time and start complementary feeding