Potty siting at 9months. I stop him.

My baby is 9months old..he has started potting siting..usualy he poops early in the morning after he wakes up..so when he show signs i take him to the potty..he is not reluctant to sit..he sits and strains his poop..after he poops he does not like to sit in potty..only condition is that he wants to see his poop..after that he wants to leave potty..he understood that the potty is just to do with potty not for playing ..now for the last few days..when he wants to poop he calls amma and make straining sound to make me understand he wants to poop...i have not potty trained him but not only potty sitting but he him self trained.so now i have read potty training should start only From 18months not early..even though i have not taken efforts he him self is in the track of potty training..should i stop him..and wait till 18months