Your child is getting better with movement and coordination. Coupled with this is his growing sense to act independently. By now he is likely to sit on his own without any help provided.
What you need to know
At this ponit, your child observes and learns a lot from the environment. He also wants to imitate how adults behave. 'I can do it myself' runs in his mind and he challenges himself. He may struggle a bit and figure out its solutions. You will be surprised to see him sitting on the chair without help or trying new challenges. Encourage your little one as he tries to do things on his own. Be there for him but offer help when you really think it is needed or if he asks for it.