Child Getting Rash On Lower Back Which Is Warm To Touch. What Could It Be?

Good day, thank you for your time.
My 10 year old son has developed a rash which was mild but now is getting worse and is warm to the touch.
The rash started in his inner thigh area, then his stomach and today he has two perfect patches on his lower back near his kidneys. He is non verbal and has ASD with motility issues so he is unable to tell me if it hurts etc.
At first I thought perhaps he is sensitive to the soap, detergent, somethingone he ate etc...but even after removing such the rash seems to be getting worse. Any input as to what this could be is appreciated. Thank you.