We are in the same boat as you basically. he's in the 15th percentile for height, but only the 2nd percentile for weight. he weighed only 13lb 13 oz at his 9 month appointment. He was gaining well until his 6 month appointment, then he dropped off. My pediatrician didn't seem worried, but of course, I'm concerned.
Here are things that I tried doing, which seemed to help as he gained over a pound in 3 weeks:
Try to stick to high calorie solids. (yogurt, avocado (a whopping 45 calories per ounce), cheese, meats, bananas, sweet potatoes).I double check to see how many calories per ounce everything has, to make sure I'm not giving him something really low in calories, like apples, carrots, green beans and squash. If I do give him something low in calories, I mix it with oatmeal cereal.
If you give oatmeal or rice cereal, mix it with a high calorie formula or pump your breastmilk and add 1/2 teaspoon of regular formula powder to 3 ounces (89 mL) of pumped breast milk.
Add butter or oil to food - 1 teaspoon per every ¼ cup of food