From the time he can support his neck.
Put him on potty at fixed time daily so he knows how to clear his bowels. Also, it's to let him get used to sitting on potty. Some kids will be resistant to sitting on potty later on & only want to poop in their diapers (my friend's kid is like that - only wants to poop in his diapers, will ask for diapers when he wants to poop).
It helps alot now that he's older & has to attend class as he'll poop in the morning when he wakes up and poo at night before he sleeps. It's also a good habit to let them get used to the potty or toilet bowl (I used toilet bowl cos too lazy to buy potty at that time).