Try walking your dog outside to potty before entering the store (a car trip can be exciting and make the dog need to potty). Make sure he knows key words indicating potty, peeing, and pooping. When I'm walking my boys and I don't want them to pee on someone's mailbox post with pretty flowers planted at the base
, I tell them "no potty on the flowers" and they listen. You could try telling your boy, "no potty in the store" when you enter. If he does, say "uh uh uh."
I believe it helps to always praise for going potty in the correct place, including outside (my boys are pad and outdoor trained). Every single good potty, I praise profusely.
It is a challenge when dogs smell other dogs, or where other dogs have gone potty. The first time I stayed in a hotel with Max, he smelled where a dog had obviously marked on the wall. He went to lift his leg, but he looked at me first as if to ask, "is this okay?" I gave him a firm "no" and he went right to his piddle pad. Keeping a very close eye on him during our stay, I know he didn't try again.