I think finding a new pedi may be a little drastic. The cereal thing is more of an old school mentality so that's probably the issue. I would express your concern with your pedi before making that decision. I wouldn't do the cereal though. If he's hungry, I would just give him more formula like you've been doing. He's probably hitting a growth spurt of something and just wants some extra right now.
Old school advice is what you put up with from your mother and grandmother because they have no reason to know any better. You want the person responsible for your child's care to know and understand the medical reason behind changes in thinking. There are lots of things doctors didn't have to know, or felt different about, years ago. The point is they need to stay current.