How old is your DS? I was unable to put my DD in a cradle hold until she was 3 or 4 months... I instead held her upright, tummy to tummy, with the top rail of her sling behind her head or around her shoulders when she was able to control her head better.
The Maya Wrap site
has a video you can download and watch to help with positioning.
I'm not familiar with the sling you have. Is it padded or unpadded? In my experience, a padded sling is easiest to use with an older baby. We have the Maya Wrap (unpadded) and have been using it since DD was a week old (she's 6.5 months now). I love it!
Good luck, and keep trying!