You can take vitamin C to bowel tolerance. Meaning if you get a little "soft" (sorry, tmi), then back off a little on your daily intake.
As far as solids go for your baby, like already said - you should wait until they are 6mos old. You can skip the cereal altogether, which isn't much more than empty calories fortified with iron that your child doesn't need because they are bf. I'd go right to the fruits and veges. Smashed avocados are excellent brain food for babies!
And I ditto the "time to get a new ped" comment. It doesn't sound like you are getting the best, or even the most updated, advice. Have you tried the "Finding My Tribe" forum for a good referral?