8 months old should be able to sit unsupported , a child starts sitting with his own support ie: with support of both his hands by 6 months and without support after one month of doing this.delay in sitting can be due to
calcium or
vitamin deficiency specifically
vitamin D.motor development of child needs to be checked with his fine motor skills like how does he hold on to objects,has he started transfering objects from one hand to another.then is he socialising with mummy and all other aspects ,if child is otherwise normal but the sitting is delayed and the birth history is ok then supplementing calcium and vitamin D might help.
Next Steps
iron deficiency anemia and rickets are common in this age group and present as delayed sitting ,so get screened for them