Agree with Martina. If iron is the worry have a look at your LOs toe nails, if they are very very flat this is a sign of low iron, couple this with your LOs skin colour...if your LO is really really pale, there might be some iron worries.
Personally though, no I wouldn't fill your LO up on processed cereals, however there is one cereal which I find fab for my kids and myself (we are gluten free currently) and it's quinoa (keen wah). It's like a superfood, brilliant source of protein and nutrients, excellent for breast feeding mummies

I would give it a go, mixed with roasted veggies, we love it! xx
I find doctors advice is generally good, BUT they have their own opinions and axes to grind, so a grain of salt is required too. I like pps believe that initially it's about variety and textures and less about "filling them up"...
Good luck sweetie xx