Background first:
Breastfed babies don't need cereal (actually, most formula-feds don't either). It was designed to teach a baby how to eat when you were told to start your baby at or before 4 months and they still had a tongue-thrust. It's iron-fortified for formula-feds because formula doesn't have easily-digestible iron like BM. "They" have recently decided that you can start with meats if you want because breast-feds can use the extra protein, so don't rule out the nasty beef and gravy by assuming you shouldn't start with it.
SO, skip the cereal and just start trying different foods one at a time. Avacado is GREAT!! High in fat (good fat) and pretty bland. BM is very sweet so the old "don't start with fruit because it will teach your kid only to like sweet foods" doesn't really apply to breast-feds. There have been a lot of studies done recently. Next time stage ones are on sale, get one of each jar and just start trying them one at a time, you are bound to come across something he likes.
Remember: BM is all a baby needs for the first 12 months. Introducing solids is about teaching them to eat and like solids, not getting nutrition. Don't worry if he doesn't eat much, if he's peeing, pooping, and sleeping well, he is getting what he needs. Personally, I tend to avoid conversations with the doc about eating and sleep because he and I just don't see eye-to-eye on the subject and I feel confident in the research I have done. "Is he sleeping through the night" "Yes"...when he's not up to nurse every 3 hours but that doesn't count as long as he goes right back to sleep...
I know plenty of women who breastfed until 7 or 8 months because their baby showed no sign of interest in food and still after that, the baby only ate a couple ounces each feeding. Don't worry, as long as he continues to gain weight and you aren't pushing to wean early, he'll be just fine! :)