I've never experienced this. If that is the case I will concede that what the OP's son is doing could be irritating. I still am not convinced that a person being short makes them more responsible to watch where they are going than other people. There are always children in the grocery store. It makes no sense to not be just as aware of them as the adults.
A free range toddler does not tend to move in patterns.
A few years ago, I was at an amusement park. There was a toddler running at me, so I moved right so I'd was out of her path. The child moved into my path. So I moved again and again the child moved in the same direction I moved in. Since I could not predict the child's movements, I just stopped and stepped aside, out of her current path.
Said toddler still ran smack into my legs and wound up on her tush, crying. She ran into me, I was not moving and did not walk into her. But *I* was at fault according to the parent and was told to "WATCH OUT". I WAS watching out, and did what I could to avoid a collision.
Add a shopping cart and a distracted shopper and the OP's child *could* wind up injured. Carts (especially full ones) do not stop on a dime.