You've had lots of similar responses so here's something a little different.
I had supply issues with my son when he was nursing and he never really recovered from the "shrink" he experienced before we realized it (he lost weight). Consequently I felt like it was my fault that he was small and made it my mission to beef him up as much as he would eat. I made sure everything I gave him was healthy but high calorie. Here are some things I fed him:
Yobaby yogurt- one of the only whole milk yogurts out there so there's fat in it
Beech-Nut used to have a line of food (can't remember the name) that had egg and cream in it in addition to the fruits and veggies so that added some fat
shredded cheese
tofu
edamame (soy beans)- once he had enough teeth and the facility to chew
I also read the nutrition info on the baby food jars and made sure everything he put in his mouth was healthy but also had some fat in it when possible (almost all baby food fruits and veggies have no fat). If your son gets full from eating instead of drinking and he's eating low-fat foods (which is easy to do with baby food), you might want to try giving him formula before feeding since that's where most of his nutrition is coming from right now.
My son moved to exclusively eating table foods late (maybe around 16-18 months) so he spent A LOT of time eating baby food. He was born at the 75th, dipped to the 5th (around 4 months of age) and is now (almost 3 yo) around the 60th. Who knows if what I did helped with that or if that was just where he was going to be anyhow but I feel like I did what I could and that makes me feel better, know what I mean?
My daughter has moved to solids much more quickly than her brother did (she's 9 months and on table food almost exclusively). Consequently, we have lots more options with her as far as feeding goes. I use a food grinder with her so she eats what we eat at some meals, another way to add fat since an adult diet, a healthy one anyway, has a balance of fat, sugar, vitamins and minerals. We have a Munchkin brand food grinder that costs about $10 at Target.
As other posters have said, if your ped isn't worried, try not to too. I made these suggestions in case you want to try them. Doing these things with my son helped me feel like I was doing something to help him.
HTH-
J.