I'm not so sure those are second hand. They are solid commons, which look pretty much like that new, especially if they've been on an exposed palette for a while.

He seems like a smart hoddie, so he's probably go about 1/4 of a sheet of plywood balanced on a wood horse with a notch for his arm/shoulder. Lean the bike up against the horse and you can stack the bricks like this, butted against the plywood in about 2 minutes. The bricks will over hang the plywood by their own friction and weight. shoulder under the plywood and you are only supporting the wood, nut the brick on the top of your shoulder, so no pain or stress to your body. Since you started out vertical, all the weight is directly down on the chassis (probably will bend the rims before too long though). Now you don't need to do much except supply forward momentum. With a weight this size, it will warn you when it starts to shift of the vertical long before it topples, and you've got handlebars that aren't being torc'd to compensate easily. If he were to lose control, he can dump his load easily by turning the wheel towards himself, and then it's a minor pickup job.

First hoddie rule. Everything you touch here is too heavy for mortals. Don't lift with the back, lift with the brain.