Wow, so many great suggestions Thanks!

I don't think DS is bored, he has plenty to play with inside and outside including his own quad bike and mini tractor (push pedal thing) A water maze activity set and the list goes on... He escapes because he likes to feed the working dogs... once he lets the working dogs out he then takes them for a walk...
The other reason he escapes is because he can see things going on over at the work shed and wants to 'help' He has gotten as far once almost starting a tractor ( a real one!)
He's just an outdoor boy,
He went out with DP one day on a water run (where you check all the stock water and troughs etc) and DP was trying to put a fan belt on a lister motor, he had his eyes off him for about 20 seconds, turned around and saw William sitting on TOP of a tank playing in the water!

Without tooting my own horn, DS is a very intelligent child who is very advanced both intellectually and physically... unfortunately his perception of danger is not so advanced...

I honestly think what I am trying to do it a little bit different to tying a child down to stop him touching something dangerous! I am trying to protect him from the dangers of what is outside the fence, not physically restraining him to the chair...

The electric fences I have looked at come in several different voltages, the one I am looking at says on the box that it is SAFE for children and pets, The voltage of such fences is similiar to a medium setting on a TENS machine.. so there you have it, it's not going to hurt him, but it might be uncomfortable whilst he is trying to move it to get over the fence...

In saying that though, I caught DS one day with the working dog training collar once, holding it against his tummy and pressing the button on the remote! He thought it was hilarious!

Addition: We do have some more child safety locks coming, slide bolts and what not, so that should keep him inside... so hopefully that is going to help a bit!