|How much does your DS weigh? Is he getting anywhere close to the upper range for the S Sandys?
I had a super-pooper who would explode out of any disposable, and found ME fantastic. I still did have the occasional leak, though, so maybe some kids can overwhelm any diaper.
One thing that I did find was that I had to move him out of the S Sandys earlier than the suggested weight. It's been a while now, but I think that I left them behind somewhere around 17 or 18 lbs because after 4.5 months of no leaks, suddenly he was leaking up the back. They still fit fine, and were not even on the last setting, if I remember correctly, but suddenly those leaks, and I just felt like maybe they were a smidge too low in the rise to contain things.
I know that that is not the usual experience with the S Sandys (my DS#2 could wear them until over 20 lbs, and most people find this).
I used mostly OS and wasn't having those leaks with them, and when I started using the L Sandys a bit before 20 lbs, no leaks with them either. Which diaper fits best exactly when can really depend on the build of your baby.
I know that's not really specific advice, except perhaps if DS is chunk to move him to a bigger diaper.