Oats/porridge is an awesome food for babies! it's low GI and much better than rice cereal, or god forbid, weetbix. It doesn't matter what you feed them and when you start feeding solids, but the transition from purely breastmilk to solids is always going to take an adjustment period for their little digestive system to get used to the bulk in their bowel etc. It sounds like he's doing OK to me.