Help 5 year old has large circle bum rash the size of side dinner plate what is it?

[nettermum]

Hi

I just noticed that my 5 year old has a rash on whole of bum the size of a side dinner plate. It's not red its a different colour to his skin though it's like someone has drawn to circle to cover that area meaning the circle is distinct. There are no scaly, red area. The circle has a slightly darker ring and very slightly lighter after thin ring.has anyone seen this with their kids and if so what is it?

Thanks

Has been sat on the loo for ages?

Can you take a photo of a small part of it (not the whole bottom obviously)