non-twin question but need advice

So I have a 3 year old son who's being potty trained. He has gotten peeing in the potty down packed. Every time he has to pee he goes without anyone saying anything but......the problem I'm having is getting him to poop in the potty he is still pooping on himself i mean he will run to the pot to pee and then 10 mins later he has poop in his underwear and we have gone completely underwear no pull ups or anything just strictly underwear. Can anyone give me advice on how to get him to poop constantly in the potty?? Please I'm a stinking sinking ship lol.