Hello & welcome, I am Dr. B, a licensed veterinarian & I would like to help you with your wee one today.
Is the diarrhea settled now? Was this area of skin subjected to diarrhea scalding?
Does the rash cause irritation or itchiness for him?
Any pimples, pustules, scabs, ulcers, crusting, scaling, or sores with this?
Can you take a photo of this rash? If you can do so, this will let me see what you are seeing. To post them online, you can use the paperclip icon on the tool bar above your text box. Or if you can't see that on your computer/phone, then you can post them on any other site (ie Flickr, Photobucket, imgur, etc) and paste the web link here for me to check