I buy my son slims at JC Penney. He's a 16 Slim. I get him jeans as well as dress slacks. I purchased Izod pants for him for choir - they had to be black. There were also khaki Izod pants there as well.
Another option would be to find a uniform store, i.e. for private schools. My son was at a private school for 4 years and we used Flynn and O'Hara. They had pants in every size, shape, and color.
I hate the pants with the elastic to make the waist smaller. He would have to tighten them so much that they would look stupid....and they would be very uncomfortable because so material was gathered.