My neighbors baby is 3 months and 3 days old, his neck is yet to be stable, his nose is flat. Is it normal for a child of that age to still have an unstable neck? Sometimes when he lay on his stomach, he tries to raise his head, he changes the position of the head from left to right but when in an upright position, the head is unstable, it could fall down, sideways, or backward without a support
For further explanation see a developmental pediatrician or pediatric neurologist to check your baby.
Kindly note that neck control is achieved by 3-4 months while sitting milestone is achieved by 5-6 months.