Sitting up with no support

Well I have been working with Kale for awhile on sitting up. He would sit like a gorilla, is what I like to refer to it as, and he couldn't do it for long. I kept working with him and then the other day I had seen people put their babies in their crib in the corner for support. Well I know Kale LOVES his mobile so I thought I would give it a try. I changed his diaper and put him in his crib. He loved sitting in his crib! It was so funny to watch him look around with a HUGE grin on his face. He watched his mobile and just looked around and was content in his crib for the longest time! Well then the next day I was sitting in the floor working with him again, like I always did everyday and he just sat up by himself. It was amazing and he did it for a while before falling back. Well I didn't expect him to sit up by himself so I did not put any pillows behind him. Well he fell back and I couldn't reach him in enough time. It scared him and he started to cry and it scared me. So for those instances when he thinks he is invincible and tries to turn too quickly or tries to look behind him...his back is not quite sturdy enough for that and he falls back I like to have the pillows to catch his fall because I don't want him to hurt his little head. I am so proud of him, but at the same time seeing my little baby sit up all by himself makes me feel a little sad. I mean he is growing up so fast and will be doing more and more and soon won't need me :( I won't be able to hold him like a little baby. He looks like a little man :) It is so cute watching him play with his toys all by himself in the floor! Now that he has accomplished sitting he wants to stand all the time. So I detect soon he will be standing or crawling!