Is your LO normally fussy around the time that you feed him, perhaps tired? I introduced new foods to my LO at lunch for the first week as I found she was more alert then. I also found that she was a lot more receptive to new foods when they were mixed with breast milk or if they were sweet foods. I've also heard that breast fed babies especially like sweet foods to start because breast milk is sweet. So I am going to be mixing veggies with sweet fruits- eg. sweet potato with pear, banana or apple, squash with pear, banana or apple, etc.
Your LO should be getting most of his nutrients and vitamins from breast milk, but iron stores do start to deplete so the best way to get iron is through cereals, which it looks like you are doing already.
I'd try and introduce a second meal once he is interested and doing well with breakfast. I don't think it matters whether it's at lunch or supper, but I personally like doing it at supper because I find my LO is more hungry then and it ties her over till her evening feed. Hope that helps!