In this area I am an authority. I have two children who are both "picky eaters" in the extreme, one with GERD. They are now healthy and approprietly proportioned and in their teen years.
First off, it is not your fault. GERD (Gastroesophogeal Reflux Disorder) is to blame. Second, in addition to GERD, my chidren are also on the autistic spectrum and therefore have sensory issues as well. There are certian textures they simply can not tolerate. By the way, if your pediatrician has diagnosed GERD, ask for a referral to a pediatric gastroenterologist. Dr. Michelle Winesett handles my daughter and has been very good.
My pickiest eater had such a problem (for reasons to complicated to go into here) that she wound up with total oral aversion when she was a baby and was tube fed for 7 years. She had a heart defect as well, and it was absolutely critical that her weight not get too low. She has now been tube-free for 7 years.
You are trying to put weight on your child. The entire society is against you. Everything your child sees and hears will tell him he is fat and to try to loose weight. Your first task is to convince him otherwise. Even at school they will preach weight loss to him. Talk to his teachers and explain that it is important that your child GAIN weight. Ask them to help you accomplish this. Also, it is easy to find foods that are geared to loosing weight. It is hard to find nutritious ones that will add weight, but it can be done.
I know you want your child to eat healthy, but your primary concern is to give them calories, not to feed them a perfect mix from the food pyramid. Your child's food pyramid is going to look more like a flying saucer for a while. Get used to it. You can fix that later. In the meantime, Wal-Mart carries some chewable Equate Milti Vitamins Complete with Choline. Not all vitamins are created equal, so compare the labels.
First off, add butter to everyting. My daughter liked those little Chef Boyardee microwavable one-pot speghetti things. Add a pat of butter, then heat and stir in. Veggies: add a pat of butter. Chicken: brush with melted butter. Sweet potatoes are wonderful for being able to mix in and slather on loads of butter (maybe with a dash of cinnamon if your child likes that).
Second, get the child to drink milk. Not skim or fat free milk, but whole milk. Not chocolate milk, most of it is made with skim milk. Add just a bit of half & half cream to each glass. And explain to the child they can not choose skim milk at school either. Also, many kids with eating problems will drink rather than eat. They will fill up on drink because it is quick and easy and satisfies their hunger now without the labor of eating. Don't let the child drink before meals. Make sure they have eaten a reasonalbe portion of their food before they get a second glass of drink. Stay away from sodas. Carbonation can make them feel full.
By the way, carbohydrates are your freind. And so is butter. And so is cheese. As all of us know who are trying to loose weight, there are some really delicious foods out there that will put on pounds. Super Wal-Mart has some wonderful frozen Great Value cheese toast. Wal-Mart also carries some individually wrapped Sargento Colby-Jack Sticksters. Pizza, pasta, cheese, garlic toast with lots of butter.
Try Pizza Hut's all you can eat lunch buffet (11:30am - 1:30pm, even on weekends). Doesn't cost much and the child can pick and choose. If you can, try to get them to eat pizza with lots of stuff on it, like ground meat and bell peppers, not just plain cheese. (Keep an eye out and don't let them get the ones with jalapenos!) Maybe one slice of loaded pizza, then they can have one plain if they want it.
And, of course, there is desert. Try home-made from scratch banana pudding complete with bananas and Nilla wafers. Sara Lee has some to-die-for Hershey's chocolate cake. You can order it at Denny's, which has a Kid's night on Tuesday evening where kids eat free (cake extra, of course). They store it cold, so just make sure they pop it in the microwave for about 10 seconds before bringing it to the table so the chocolate is just almost melting. You can also buy a whole cake from them to take home.
There is a good chance your kid likes crunchy and salty things. Even junk food can be your friend. Pepperidge Farm Goldfish, Sunchips, potato chips, Cheetoes, buttered popcorn. Old feeding therapy trick: Try negotiating with your child to eat what you want. You eat one bite of what I want you to eat, I give you three goldfish. Sometimes I have to go through a whole meal that way with my autistic son. Interestingly enough the "gaggies" often disappear when he has an incentive like goldfish. I've even done this with cookies. My son likes the Soft Baked Peperridge Farm chocolate chip and Dark Chocolate Brownie varieties. Break into fourths and use as an incintive at supper. So my child is eating cookies at supper. At least he his eating the foods I want in between bites of cookie.
Sometimes it also works to distract him. If I can get him to do an activity he likes, I can get him to eat a bite while he does it. He's concentratring on the activity, not the food. Again, eat a bite, do fun activity for a couple of minutes, eat another bite, do more activity. It is work for you, but it works.
Don't stop trying to expand their menu, but don't make a battle out of it. Try one bite, get the goldfish, or Cheetoes, or Sunchips, whatever they like. If he gags on it, wait a month on that food and try it agiain. Right now his menu is actually shrinking again as he enters adolesence. His tastes are changing again, as ours all do at that time. But I keep trying.
Your last line of defense is Boost. There is a variety to add calories. Boost is a canned drink that can be found in the pharmacy section. Or PediaSure. PediaSure contains all the nutrients that your child needs. My child lived off almost nothing but PediaSure for 5 years. Have them drink a can between meals.
Good luck! You are NOT ALONE!