G.G.
My daughter has always been tall and thin (underweight, in dr. speak), so I learned to take in clothes so they would fit her the way they should. It's not that hard to do, and it's nice to be able to buy clothes for her "age group," so she doesn't look like she's in "baby clothes." But adjusting the clothing is the easy part!
Dealing with doctors can be hard because they are constantly looking at charts and books, and finding that my daughter isn't "typical." Well, I KNOW she isn't typical!! She wieghed less than a pound at birth, and wasn't expected to live, let alone walk and talk and learn!! So when I found a doctor (in Kansas ... sorry) who said, "Your daughter has obviously not read the textbooks," I knew we were in a good place. Her approach was to watch my daughter's health and energy levels ... if my daughter was happy and energetic, doing the things that babies should do, and if she was healthy and not catching everything that came down the pike, my doctor was okay with that, and so was I. We learned to know what was typical FOR MY DAUGHTER, and THAT was our guideline, rather than the charts and books. We tried adding calories to her food and bottles, but then she would just take less overall ... it's like her body knew what she needed.
So, based on my experience, I'd say to follow your instincts and not worry about the weight. Your son will eat what he needs and, as long as he maintains his health and his energy, he will be fine.
Good luck with school and enjoy that little boy!!