I would totally go the fence over extra locks. I grew up in Northwest WA and spent a lot of time of stations with friends who lived on them and etc. If there is a fire and you have to spend extra time with locks it's very worrying. A shock wouldn't hurt him - as said above - it's a stingy tingle, from my experience (on a dairy farm).
I don't see the fence as child abuse at all, especially considering your circumstances. What is the cost of electric when compared to one of those big cylone fences with the large overhang on the top? Maybe a bit cheaper and just as effective.
Of course that wouldn't stop him taking to it with the wire cutters in a few years! :-P
If you had a small enough weave in the fence wire, it may even keep a couple of the Blake's out!
Do what you need to. I highly doubt he will grow up saying "I feel abused becuase of Mum's fence". As you said - finding him down a well, dying from a snake bite or wandering off and getting lost and dehydrated in the bus would all be HORRIBLE options.

