I wouldn't stress it. Give it a few days and then offer again. The cereal's not that important yet - by about 9 months, it's helpful to have the extra iron in it, if they haven't started meat by then, but at this point, it's just to get used to different tastes and textures.
As for adding fruit - he's old enough to try - just make sure to start only one new thing at a time. For the first year, milk's the main source of nutrition anyway!