They can be Diaper Rash
or even a fungal infection
. Do two things, thoroughly clean his buttocks with water after he passes stools everytime and then don't forget to dry the area with a wipe after that. Since the weather too is humid nowadays, even slightest moisture left can cause a rash as your child seem to be suffering. Then apply an oinment which is a mix of some mild steriods and anti-fungal 2-3 times a day for a week or so and your baby should be alright. Good luck.