Hi, First of all , you say your little 4 and a half yr old id asking for a rabbit, and you may want to eat it!
....i agree with TMG, its got to be either a pet, or food..By all means, keep rabbits for meat, but i would think it would be better to seperate them in his mind, especially given his young age.
As regards rabbit proofing a run, its not hard, just use weldmesh, and watch out for tunnels. If they look like they are burrowing, put mesh on the base of the run also. As regards electric running to the shed...im not sure why you would need electric ?, and there is no welfare concern with rabbits kept outside, except in the coldest of weathers. As long as the rabbit is in a warm, leakproof cage with shelter and somewhere free of draughts to sleep, and it should be fine. A large wooden hutch with a bedding box will be very easy to bulid (or buy). There really is lots of info out there, have you been to the library?
Also, be very careful with your sons allergies. Inside or outside, Rabbit fur is very fine and gets everywhere (up your nose etc), and if you couldnt pet them because of the fur, you would have one very unhappy bunny!