What is wrong with his butt?

Doctor can examine: Needs in-person or video exam by his doctor. Babies butt and thigh rashes can be eczema, which appears as reddened, rough, dry skin areas. On worse eczema areas, the skin will get tiny cracks that ooze moisture. Rashes can also be candidal yeast rashes, urine or chemical contact rashes, bacterial cellulitis rashes, etc... Changing laundry detergents usually doesn't make a difference in the rashes.