OP, my severe picky eater wouldn't touch anything like Pediasure. Will your kid drink it?
There's nothing magic about Pediasure. It's sugar water with some oil thrown in, and a little milk and soy. Plus a vitamin.
Water, Corn Maltodextrin, Sugar, Blend of Vegetable Oils (Canola, Corn), Milk Protein Concentrate, Soy Protein Isolate, Nonfat Milk. Less than 0.5% of: Natural & Artificial Flavor, Cellulose Gel, Potassium Chloride, Magnesium Phosphate, Calcium Phosphate, Potassium Citrate, Tuna Oil, Calcium Carbonate, Potassium Phosphate, Choline Chloride, Ascorbic Acid, Monoglycerides, Soy Lecithin, Cellulose Gum, Salt, Carrageenan, Potassium Hydroxide, Inositol, Taurine, Stevia Leaf Extract, Ferrous Sulfate, Monk Fruit Extract, dl-Alpha-Tocopheryl Acetate, L-Carnitine, Zinc Sulfate, Calcium Pantothenate, Niacinamide, Manganese Sulfate, Thiamine Hydrochloride, Pyridoxine Hydrochloride, Riboflavin, Lutein, Copper Sulfate, Vitamin A Palmitate, Folic Acid, Chromium Chloride, Biotin, Potassium Iodide, Sodium Selenate, Sodium Molybdate, Phylloquinone, Vitamin D3, and Vitamin B12.
So if your child is willing to drink it, it's a source of calories and protein and vitamins.
But you could also try a milkshake, or a chocolate milk, or strawberry milk, or a yogurt drink.
I experimented and found a recipe for a milkshake that my severe picky eater liked, and a smoothie recipe as ell... very gradually adding some fruits blended in. He also drank a lot of hot chocolate (to which I added one small egg yolk).