Yes, the iron in breastmilk is absorbed better than most iron, but I will say that a lot of exclusively breastfed infants are a little anemic at one year. I'm a dietitian for WIC (women, infants, children nutrition program), and I see it more than I would have thought. We don't check the mother's iron at that point, it would be interesting to see how many mothers of anemic babies are anemic themselves--but most of the mothers weren't anemic postpartum.
I don't think you should stress about it too much, but it's not a bad idea to keep giving him a little bit of cereal, or else other iron rich foods as he gets older. If he doesn't like plain cereal, try mixing it with fruits and veggies that he does like. (This would also boost his calories, if you are concerned about that) As someone mentioned upthread, beans are a good source of iron, so hummus is good. My year-old niece loves black beans, so that would be a good idea when he is ready for finger foods. You could also give him baby food meat starting around 8 months, you can mix that with fruits and veggies too if he won't eat it plain (I wouldn't blame him for that, pureed meat is kind of gross!) Like I said, I don't think you should worry about it too much, but it is good to be aware of.
I think what you are doing is great. Breastfeed him when he wants, and then feed him solids a few times a day and let him eat what he wants. He'll be fine. Also, I would like to point out that 30th percentile is totally within the normal range for weight. Anything between 10th and 90th is fine, and unless he's dropped in percentile, he is just fine and there is nothing to worry about.