So lately DS continually tries to stand up in the Ergo

No matter how tight I try to make the waist strap, he sticks his feet down in it and stands up on my hip bones. I'm not using the infant insert and have just been rolling up the blanket for under his butt. He isn't whining or anything when he is in it, just acts like a little jumping bean by wedging his feet down into the waist band. Any ideas? What am I doing wrong??
Re: Ergo Help