Hard situation, kids are so darn persistent at doing things they know they shouldn't. You almost needed to have the fence built the other way around so that the cross bars are on the outside and all you can see is the up and down of the pailings so he has nothing to climb. Would putting something like a tarp over the fence so he can't get a hand/foot hold work? Remove the bottom rail - don't know if a fence can be structurally sound with out it but you did say you were desperate!