He is potty trained for a few months now and has no issues using it when he needs to go for a wee. He was doing absolutely fine when he wants to go for a poo, but one time he became constipated for a couple of days and when he went to have a poo, it was painful for him (his eyes teared up). He was screaming when I told him to sit on the potty as I knew he needed it.
Since then, he just goes for a poo in his underwear and not in the potty. He only uses it to wee in. I keep on reminding him to go on the potty when he needs a poo but he doesn't. I get no warning that he is going for a no2, he just does it and then he says "I done a poo poo mummy" and immediately I know he has done it in his pants.
Any ideas what I can do about this? I know it isn't his fault bless him but it's getting a bit too much, especially as he goes 2-3x a day and in public I have to quickly find somewhere discreet to sort him out! I continuously remind him to please go on the potty but it obviously he doesn't really understand/take it in.
Help! 