Get that child to a doctor ASAP! Preferably without offending his parents, but that is a risk you really will have to take.
If his bottom is really sore, which I bet it is, take his nappy off when he is having a nap. Lay him on a layer of clean soft towels with newspaper under, and as boys have a long range lay a clean nappy over him.
Here in Denmark, small pieces of lint bandage used to be dipped in cold, boiled water and put on the sore places on a baby's bottom before the nappy went on to alleviate the discomfort of a nappy rash.
The yellow motions may be normal for this child, but unless a doctor examines him, you will continue to worry, and the nappy rash is a problem too.
I would be worried too, that all these dirty nappies might lead to an infection of the urinary tract or to an adhesion of the foreskin.