Weird mark when skin is red

So I'm not sure if this is even really going to make sense 😂 and I don't have pictures of it yet. But ever since my son was little, he would get this white shape in the exact same place and exact size above his right butt cheek. It's not visible all the time, just when his skin is red or he takes a bath. When I first saw it, I thought it was an allergic reaction but it isn't. It literally looks like a burn mark right after you get burnt and your skin turns light because it dies and the area around it is red. It's not raised at all, it's just a white kinda abstract rectangle shape. I'll see if I can get a picture during bath time tomorrow because it's so bizarre. It literally doesn't do anything or bother him, it's just weird. I figured I'd see if anyone could tell me what it is? Maybe a birthmark or something?