Chances are if you child is 'picky' they will not drink Pedisure. Supplement shakes often have a metallic taste and a weird texture. My nephew is picky (texture,smell expecially) and has been since he was a your son's age. He never would touch the any of those type of drinks.
His mom even tried plain old milk shakes. No go.
Fact is that he's almost 17, and elite athlete and still picky and has a limited diet. He is cut like a male model, fast, has uncanny periphial vision, and can jump higher than guys a foot taller than him. He is only 5"8 but my dad didn't top 6 ft till he was 20 and the doctor says he still has room between his growth plates so will grow more. Unless your doctor says there's a problem, just let him be.