oatmeal/porridge oats will soften hard water, I would do that 1st of all. Tea Tree oil, just a couple of drops will help with the soreness and healing too. It is also an antiseptic. Lastly, I would put him to bed in pants, and a big blob of vaseline on his sore bottom over night. Hope he feels better soon.
Separate the egg and whisk slightly to break it up a bit. Keep in fridge to cool it down. After every bout of diarrhoea and before bed take a clean cotton pad and smear some egg White over the sore skin. Use a clean pad each time you dip it in egg White so that you keep the egg clean. Allow the egg to dry and the skin before putting pants on.
The egg forms a barrier on the skin and the protein speeds u the healing process. As the egg is cooled from the fridge it is very soothing for sore skin too.
I use it for my dd who as a baby had awful diarrhoea every day due to medication and specula feed she was on.
You use it until the skin is not sore any more. which in my experience is only about 24 hr. Throw out the egg every 24 hrs and replace new one ready in the fridge.
I am a nurse and have used it on patients with their consent. It used tp be used a lot on the past in nursing, but the use of it was changed to other sorts of creams not because it did not work , but because of storage problems of the egg in hospitals and risk of allergies for those patients with undiagnosed egg intolerance.