My son has an odd rash on his butt and I have no idea where it came from. I would post a pic but I'm…

My son has an odd rash on his butt and I have no idea where it came from. I would post a pic but I'm not sure if I should post a pic of my son's butt. I know there are perves out there. It doesn't look like a normal rash. Its on his lower back and just below his butt crack. I know he has my sensitive skin but if he has ecsama like his daddy, we have to clear it up