I've tried NIDO for my little one but somehow he refuses to take it no matter how much I convince him to, what brand should I try next?

How about Bonakid? I heard it tastes good for kids. However, as far as I know it contains a significant amount of sugar, which probably explains the kids' love for it. It won't hurt to try, anyways. Of course, you can always ask your pedia for more options.

What's your baby's age? I've read that Lactum and Pediasure are good for picky eaters so why not try it? You may also check other brands like the ones from Enfagrow line, Nan line and S26 line.