My kids loved avocado at that age. Banana, sweet potato, apple sauce, crackers (there are SO many kinds!) Quaker Instant Oatmeal has lots of flavors, toast with lots of butter or jelly if he's eaten those fruits already, YOGURT! My kids have two or three containers of yogurt a day and it's SO good for their little tummies. OH! and they can dip graham cracker in the yogurt. They love that! I get the yoBaby yogurt... start really mild like the vanilla and banana flavor. Pasta... MaC and Cheese... my kids LOVE that. Um... soups? Muir Glen makes great organic soups and my ds loves them. (18 months) Lasagna... spaghetti... chicken nuggets cut up really small? I mean, REALLY small?
Peas, corn, creamed corn, mashed potatoes?
You can sit with him and say, chew, chew, chew! That always helped to remind my boys to really chew. But seriously, don't worry about percentiles.
Are you still nursing at all? Because that can really help make sure he's getting all he needs, too. If not, don't fret. He will be just fine. It's hard, I know. My first ds was in the 5th or lower percentile for weight. He nursed to 19 months and was really picky for a while. Then sometime in his second year he just GREW! Now I have an 18 month old ds2 who is in the 98th percentile and just huge! Still nursing, but he also eats everything. They are all just so different. Don't worry. Enjoy experimenting! :)