I Have A Toddler Grandson Who's Always Constipated. We Fed

I have a toddler grandson who's always constipated. We fed him fiber rich foods like papaya but it doesn't help much. Recently ,we gave him pediasure and he loved it. He finished almost 2 bottles a day for a couple of days ago. But he's still constipated. Then I red a blog that pediasure can also cause constipation. If so, is there any other nutritious drink that can replace pediasure?