Everytime after he bath his back will be like this ? Anyone any idea why or have experience before ?

Have u looked at the soap u r using for him? Could he be allergic to some brands? Can try something more ex like Sebamed. Might solve the problem. Use of moisturiser will also help for sure. Do not use steroids cream excessively. Will burn the skin one. Know that because I've sensitive skin and seen the NSC specialists for more than 10 years