You can wait out for 24 hours before you take to the Pediatrician
. Since it is not itchy or red or pain it is non worrisome reason.
You can just apply oil / moisturizing cream thin layer.
This might be because of hot water that causes vasodilatation and this subsides after few hours. THe second reason if he has changed the regular soap that can be the reason. In such a case please get back to regular one.