I need advice, baby weight gain trouble

We were referred to a GI for something totally unrelated to my almost 6 month old's weight. well the doctor wants us to put extra formula in his milk to add calories so he can gain more weight. My son is 13.6 lbs right now and is in the 10th percentile for weight&length.; I don't want to fatten him up since he is very healthy. do I Have to do it? is there an alternative for healthy weight gain. I already fever up breastfeeding to help him gain weight. I'm mad that the doc is pushing for this. sorry for the long post.