Try a protein powder mix in water or soy or rice milk . They make whey protein powder. Sold at any health food store.

Soy protein (soy milk, tofu). Fish, rice, potatoes, fruits and juices might give you some ideas for a menu.

I am having the same problem with my 15 month old boy. I have been working with his docs and a compounding pharmacist to get his weight up. First- you should really minimize the soy in his diet. Soy is a phyto-estrogen and is not good for children (esp. boys!) Google "soy formula like birth control" and you will find a lot of info on this. We have started combining a rice protein, with flax seed oil (has your EFAs)in rice milk (my son can't have dairy or gluten) and have a "shake" that is about 300 calories, but is thin and goes down like milk instead of a milk shake. I give that to him 3 times a day in addition to his solid foods. I also slip in other "good for you" things- like Future Formulations Super Immune Space Sprinkles (Nat-Stim and dairy-free pro-biotics for children), Lecitin for brain development, IGg (by Xymogen) for gut health, etc. He has started to gain some weight and we are hoping for continued sucess. Wish you the best and keep us posted. (BTW- It could also be your kid's growth pattern. My daughter didn't grow in keeping with the charts- stayed around the 25% for the longest and started growing FAST when she turned 2- when the doc said her growth should slow down. Now she is 4 and as tall as the 6 year olds on her soccer team. Hoping my son will catch up too, he has been in the 3% and has other issues with gluten and dairy...)