13 months baby not able to seat. A time.

My baby is 13 months old. He born before 1 week of his delivery date with normal delivery.He started controlling his head at 6 months of his age.Everytime when we visit for regular checkup his weight,height was good as per the Dr. Expectations. Things which he is doing 1. Recognised the family members and go to them. 2. Give smile and does activities like giving high-five from both hands but one at a time only. 3. Play dhol by his both hands at a time. 4. Transfer object from 1 hand to other observe the object. 5. If we make him stand he can stand without support for 20-30 sec. Also try to walk by having 1 or 2 steps 6. Create different sounds when playing. 7. Move through out the room on his butt like in seating position. 8. When we hold him and if he is bored bang his head on us Things which he not doing 1. He is not able to getup by his own when he is in sleep postion ,Giving kick by his both of the legs 2. Lack of coordination of hands when playing.usualy use one hand at a time.