My husband built a beautiful fence around our yard
That's a hard one, Rachel!
When my ds was about that age, he learned how to open the gate in the backyard and came around to the front of the house and rang the bell to be let in. I'm glad you're keeping a good eye on him! We were able to fix it so he couldn't open the gate but I can't think of a way that you could keep yours from climbing. (They can be such stinkers at doing stuff like this sometimes!) Would it help to have some sort of a jungle gym or something he could climb on in the yard?