First off, your milk suppply is FINE! Your baby is more efficient now, and your body is way more efficient at making only the milk your baby needs. Also at this age, your child doens't require as much foremilk,and that is what made your breasts feel full. Now he's getting more fatty milk, and he doens't need as much of it, so your body is making it more concentrated, more fatty. THAT is why you don't feel full. I don't think many moms would make itn ursing long if thier breasts felt full all the time through the first year! I know about 5-6mos my breasts regulated with both kids, and I wasn't full, nor did I leak. Its because my body figured it out, and it knew what my baby needed and made only that, nothing more.
Pumping is no indication of whaty our supply is, your baby can get out 2-5ounces MORE than ANY pump can get. And I'm not surprised a doctor would just automatically suggest formula... look around thier office. Similac or Enfamil SUPPLY thier office with clipboard, paper, pens, facilitating whole wings of peds offices! The doctors are basically PAID by the formula companies to advertise thier products and hand them out.
Again, your milk is fine. Those growth charts are made for formula fed babies, not breastfed. Breastfed babies don't fatten up like formula babies because they regulate thier own diet, they choose what they eat, how much.
Don't question your milk, as long as your son is content, happy, nursing (and he's nursing quite often so keep offering) and just relax! The more you supplement, the moer your milk will suffer.... it is supply and demand and if you take away the demand by supplementing, then your supply WILL drop. Right now its not, you just have more concentrated milk to match what your little guy needs. Formula can't do that, so stick with nursing, it will be ok. He sounds totally normal to me.