How do you train your yorkie not to pee/poo in stores? I am was so embarrased the other day when we went to a dog botique and he peed and pooed in the store!

He is house trained, he is so good at home, he only has accidents when someone forgets to let him out but when we go to the store its like we are going to a dog park, he just thinks he can pee and poo where ever he likes. I don't even want to take him to the store with me any more- any one else have this problem or have advice???