What Is The Dark Brown Patch On Back?

My son, age 46, is in the hospital at present getting over surgery for appendectomy. Since he has a hospital gown on, open in the back, I couldn't help but notice a huge dark brown patch in the middle region of his back--almost looks like dart dirt but since I know he showers daily and looks clean elsewhere, I am puzzled and worried. He is a very private person and I know would be embarassed that I had seen this. How do I approach this with him and what do you think this could be. Thank you