My suggestion would be to feed him some insect, and while he’s distracted eating them, very gently pet his backside. I‘ve done this with Bepis, my turtle, as well as two stray kittens that had turned up in my yard. I know he’s not either of them, but it could still work to a point.
It’s also good to note that Petsmart isn’t a very good company, nor is PetCo, so he may have had some bad treatment there or some other history that could have a factor in this behavior. Make sure he associates you with good things like food, and hopefully he’ll begin to warm up to you. I’d also make a note to be sure that your kid is aware that he can’t treat the beardie as a toy. I’m sure your kiddo may be aware of that already, but sometimes things happen when beardie’s are purchased for children, which can result in the poor treatment of the critter from the child. Just to be sure lol.
I’m not sure how old your beardie is, but they are very territorial, so that may be another factor. He may see your hand as an invader into his territory. As the last replier said, I’d make sure his temps are good and have an appropriate tube UVB. How big is your tank? If it’s not big enough, this may be a part of the issue. What kind of substrate are you using, also? Husbandry may play a part in this, so I’d be sure everything is appropriate.