DS (2.4) Just screams and cry's when we go out :(

[StrippedPJs]

He's been 'potty trained' indoors for about a month, after the first week he could wear pants I took him to a friends party and he's kinda scream at me or grunt and I'd take him to the potty it was great.

Then it went downhill, he did a poo in his pants and I had to change him in the corner of next as he was shouting POO, since then he's wee'd in his pants and every time he goes into a public toilet he just screams!

I've tried giving him stickers, minion haribo's clapping cheering everything all he does is scream. Since then he's wee'd in his pants and I thought he wasn't ready but the last two days he's had pants on and he's told me "wee wee" and taken them off and gone for a wee so he can do it.

I brought him a travel potty and he will go in that if he's in someone's home but as soon as we go outside to the shops etc he goes in his pants

What am I doing wrong?