Most of the bicycle trailers on the market have a max combined weight of 100lb between 2 children and gear. Your big problem is likely to be width and height, not weight. The trailers are built for smallish children and frequently have dividers between the seats.
A chariot cabriolet might work for you, since I think it allows sitting in the center position. A burley cub is probably worth trying, since it's the largest burley. I think that REI tends to have both of those in stock so you can see how your son would fit.
Trail-a-bikes are an option. You can usually get an optional seat accessory that provides a backrest and seatbelt. They come in varieties with and without the ability to shift gears.
For yourself - get the lowest gears you can, because you'll need them if there are any hills where you're riding. Even on flat land starting from a stop with a loaded trailer is like trying to start a semi powered by a geo metro engine. Riding is easy - acceleration is very hard. You'll want powerful brakes too.
You might want to consider something like an xtracycle. It isn't a traditional trailer - but he could sit on the back and rest his feet on a set of footsies. An xtracycle will probably be easiest to drive of the options. There's a new frame built to have native xtracycle capabilities that Surly will be releasing this summer.
A trailer is the safest, but the hardest to manuver and propel.
