At what month did your baby start sitting by his or her self?

My baby boy is now 5 months. He will be 6 months on Sept 21st. From the past 4 weeks, I have been trying to get him to sit up on his own. He spends alot of time sitting in a bouncer and also on his Bumbo seat so that he learns quicker. Whenever I try to make him sit up straight on the bed or on the sofa, he used to lean forward to hold himself up a bit and then quickly would fall back. From 2 weeks, when I make him sit. I now just touches his knees and then he can hold it there for a minute or two before falling back. So he has gotten closer. But if I left him alone to sit up straight, he would not be able to just yet. From the way I am mentioning his sitting with touching his knees or thighs, how many weeks more do you think it will be until he will sit up on his own?