If the fence is so low that you think he could jump it then he shouldn't be outside unsupervised anyway, so you could just move him away from it if you are out there. Our fence is about five feet as well and our dog has made no move to try and jump it yet because he already has plenty of space to run around in the back yard. Just train "on" and "off" with furniture or with pawing at the gate or fence...that least helps get it under some control. I don't know if this is true or not, but a trainer told me there isn't any really great method for keeping your dog off the furniture when you aren't there. I suppose you could use some sort of method to deter it, the same way that you would spray something bitter if you wanted to dissuade chewing, but I don't really know how to keep them from ever jumping on furniture if you aren't there.
We always make the dog "do something" before he gets to come up on the couch - like sit, or shake, or down, or some sort of way to say "please." Then we grant him permission. If he gets on the couch without permission we give him the "off" command, and if he doesn't acknowledge, we physically remove him. It gets better with repetition, but consistency is a must.