Why does my daughters bearded dragon have a dip in its back like right in front of his hind legs.?

The store we bought him from told us his growth was stunted. He's growing well and has shed multiple times since december. He was 5in. when we got him and he's a whopping 8 in. now. I'm wondering if the dent is why he wasn't growing properly. Any help would be much appreciated. We love our little beardie!