Oh boy, I've been down this road! If he'll drink the Carnation, why not give him a healthier version of that? Skip all the emulsifiers and switch him to Reliv's Now for Kids - it's much more balanced and it's full of FOOD. Kids actually DO live on it - preemies get it thru feeding tubes, the company has feeding stations all over the US and 3rd world and for some kids it's all they get. You get great support from the company's distributors (who are really well trained on an ongoing basis) and there is a money-back guarantee. It comes in vanilla and chocolate, and it's delicious. It will give him all the nutrients he needs so that whatever else he eats is a bonus. Also, a friend of mine gave it to her daughter who has severe peanut & egg allergies - after 1 year, the numbers were nearly half, and as the 2nd year anniversary approaches, she's no longer testing positive for either one! You can give it plain, in milk, or mixed into a smoothie - a good way to get some fruit into your child. Since all our food is depleted anyway, we need to supplement (says the Amer Medical Assoc), and liquid supplementation is best (more absorbable). That would work well for your son anyway because he seems to like the liquids better than the solids!
Your child may have a texture issue - mine did. It was less about flavor than about how the food felt on his tongue. Also, it takes about 8 times for a new food to be accepted by a child. So if he won't eat it 4 days after he ate it the first time, introduce it again 4 days later. It's hard to deal with waste, I know, and it's frustrating. But it does work.
I also gave my son power-packed French toast & pancakes - I put all kinds of stuff in there that he never knew about! If your son eats a good breakfast, I can give you recipes to make it even more vitamin filled. There are also some ways to sneak things into the lunch & dinner recipes. I'd be happy to give you more suggestions so feel free to email me directly.
My son was also on the small side until high school so I sympathize with your concerns. He turned out to be a lot taller than the pediatrician predicted. But then again, I put him on liquid supplements so that could be part of it. Happy to help you more!