Cool idea. What about starting with a go cart chassis to start? Beef up the suspension and such...
Air shocks are a good idea too. Not the old monroes under the old p'up but rockcrawler style air shocks.
You could also buy an old quad and gut the drivetrain from that. There are a BUNCH of cool options.
Always remember, it's not just your son that you are building this for... it's also his mom. If she doesn't like it forget about it!
Keep us posted