I'm with all the other mommies here- your son is fine, and your pediatrician doesn't know much about breastfeeding!
75th percentile for height and 50th percentile for weight sounds absolutely great to me! There's nothing wrong with being tall and slender, especially with the epidemic of childhood obesity that's going on! It's also totally normal for a breastfed baby to stop gaining weight as quickly after 6 months, and slide down the charts. Once he's mobile and burning lots of calories, the baby chub start to melt away. Formula-fed babies, on the other hand, tend to bulk up in the second half of the second year- and yet, for some crazy reason, it's the formula-fed babies who are the standard!
You might want to give this abstract from the journal Pediatrics to your pediatrician the next time you visit. Perhaps it will stop him from giving bad advice to other nursing moms! And you might want to hang on to this one for yourself- it should convince you that you are doing everything right!