I have been down the same road. Dont' walk,,,run to another pediatrian for a second opinon. I did and it changed everything.
Mine had frequent ear/sinus infections. When he was 9 months old he was not crawling, pulling up, etc. and I knew there was something off. I took him in for a second opinion after a week of sleepless night, 1 night of violent crying in pain, and all week of sporadic eating and throwing up. My regular ped had told me it was a virus and sent me home.
The other ped told me the kid had ears so full of puss and infection that it was a wonder he was as happy as he was. He put him on some pink liquid- RYNATAN- for congestion. It was just a basic decongestant. He also gave us antibiotics, a shot of something Re or Ro drug. It was a name that started with those letters. It was a strong antibiotic and cost me $50 bucks alone. He also gave me specific directions for feed, sleep, etc.
Within a week my child was eating, sleeping, and playing better. Within a month he was crawling, back to weight gain (he was a little chubby so that was not a major concern), and normal growth milestones.
For the first 3 years he continued to have infections and the only drug that seemed to knock them out was Augmentin. My other tip is to learn your drugs, what they do, best way to dose and chart your health care yourself. You don't want to become dependent on drugs, but you should at least know what you are dealing with and have a dr that will be more knowledable.
I am not saying that is going to be what happens to you, but I am saying that not all dr's know everything. We are our own best advocates. Asking questions and getting other opinions is often a big difference in our experiences.
Good luck,
ts