I completely ignore the weight gain stuff. I'd be worried about the doctor trying to force something on my kid that we would find out 10 years down the road wasn't good for him. I have a 15 month old who is twenty pounds, and has been so for at least three months. She is at the stage where she is lengthening and less fat because she is walking around and climbing on stuff, not just sitting in her high chair getting fatter....boy, was she fat! The doctor probably would have told me to stop nursing her because she was too fat- or else thought I was feeding her butter! I nursed her completely for the first six months, then introduced some solids, which I lament because I gave her too many grains at first, but now I know better.
If he looks healthy and is active enough, IMO, ignore the doctor. I really believe they mean well, but I also believe they are very confused about a few things and I blame the education provided to them by the heads of major corporations (they are on the boards of directors at the schools, and write the curriculums too).
I really like the book "How to Keep Your Child Healthy, In Spite of your Doctor" By Dr. Mendelson, a pediatrician for at least 30 years who has seen some bad trends, and enough to make sense of it all. He gives practical advice and medical/ biological understanding so you can confidently take care of your child's body and know when and how to treat at home, and when it really is time to go to the doctor.
Our family has decided not to do "well baby" visits. We also do not vaccinate. We believe they are our children, not wards of the state or the hospital they were born in, and we are not answerable to those people save for the just laws of the land. I'd encourage you to do a little research so at least you know enough about children's health not to be at the mercy of the whims of a doctor, no matter how kind and well meaning. I'm not trying to insult doctors, I know there are plenty of good ones out there, like Dr. Mendelson.