Thank you for your query.
could be due to the allergy
to cow's milk in Cerelac. As the rash has subsided now I suggest that do not give him Cerelac for the next day or two. Once there is no rash then re-introduce Cerelac and observe. If your son again develops it then the milk in Cerelac is the culprit and you will have to avoid it.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.