Withhold the apple, for three days, and offer pear, or banana or avocado. There,s no need to reintroduce cereals, but anyway the most likely culprit is barley (of the rash, if it has dissappeared after withholding cereals). If you are going to reintroduce, try one by one (not mixed together) and perhaps rice, millet or corn, followed by oatmeal in about a months time. Doing it one by one will help identify the culprit of the rash. It can be a reaction now that he will not have a year from now...
At his age, his only learning to enjoy other food and the medical reason to give it is to get more iron into your child (mostly it is because of that, which will be needed at one year and is needed after 6 months) and preparing him for eating more after one year of age. You can introduce meat and vegetables, there is a list posted here of things to avoid to which I must insist in adding now cow,s milk and no yogurt or cheese till one year of age.