your LO is only slightly smaller than mine (see stats here)
And assuming that all else is well with your LO - sounds alot to me like a difference in philosophy between your pediatrician and ours. I just asked at our DS 12 mo checkup about getting higher calorie foods into DS and the pedi said no, just keep offering him a variety of healthy choices.
When my DS was 9 mo he had only recently gotten into eating any real amount of solids, and that was also ok.
Since my DS is also small - I was actually holding back on solids a little because breastmilk is such a great source of calories and fat compared to most baby foods.
I'd personally offer to BF more often and do breast compressions to help make sure that LO is getting the fattier hindmilk rather than do something artificial like pediasure.
Hang in there!