I think colour-bond is a good idea, but it's a pain in the proverbial to put up, and we can't get a professional to come out here and do it, we are just too far out... it's also rather expensive and whilst it might be a nicer avenue than an electric fence, it is going to cost around $6000 to put it up around our entire yard, not something we could afford until we get the baby bonus from the new bubby, and that is 8 months away at least! lol... I would think by then he would be a bit more understanding about the dangers etc..
Hate to put a dampener on this one, but my DS2 sounds much like your DS - and he went right over a full height colourbond without any help. There was a retaining wall under the colourbond on the other side too making it an about 8ft drop on the other side and he did it without injury or fear.

DS2 has sensory intergration disorder - specifically vestibular hypo-sensitivities.

I know this isn't much, but my drastic measure was a child harness. He was 'attached' to me all day. Not easy as I had a baby and 3 older kids. I almost cried with joy when school started for him. At least, until he escaped class, the school grounds and walked home without anyone noticing. The school was obviously very apologetic, but that was my boy.

Interestingly, it stopped when his dad quit his job and started working from home, so they could well be onto something.