How old is your son?
If he's old enough for solids pear is a pretty good starter, also baby rice (not sure if you meant this was the cereal you tried). The guidelines say trying solids from 6 mo (my son was an exception because he was massive and I couldn't keep him filled on just milk) and when we started we tried a new fruit or veg each week. Ds found some of the fruit such as apple and plum a bit acidic so we mixed it with baby rice (made up wth his formula) and made a creamy dessert out of it.