But 30 pounds of dynamic load is indeed a bit much to carry in a pack (easier if he was, say, a sack of potatoes). What you need is a sled that is safe, sturdy, and fairly easy to haul. The key feature here is to have solid "traces"the part of the sled that extends from the sled to the wearer. These are what gives you good control over the sled, so it doesn't, A) constantly whack you in the heels (like the sled I hauled up DenaliI grew to loathe that thing), or B) skid to the side and smack into a tree, which would be very, very bad.
With that in mind, what you need is a Kifaru Armadillo sled (www.kifaru.net), which used to be sold under the Mountainsmith label and may be more familiar to people under that brand. It's an excellent slednot quite four feet long, with lots of room for a kid or vast amounts of gear. Made of super-tough plastic, it's all but indestructible. Two rigid traces attach to the sled, then to a belt/harness on the user. And they come with a variety of optional add-onsamong them, a nifty kid seat and even a full cover for the tyke, to keep him warmer (and happier) in foul weather.
The only real downside is that this is not an inexpensive piece of equipment. The sled alone is $455, the "kid kit" (seat, harness for child, and windscreen) is an extra $121, and the full cover another $53. A marginally cheaper option comes from British Columbia-based Buggaboo Buggy. Its Child Buggaboo (US$350; www.buggaboobuggy.com) is made from tough polythene and has easy-to-lug aluminum runners. Add $80 for the child seat and windscreen.