Super super quick reply as I am about to head out the door... Totally non-philosophical suggestion, but have you tried really involving DS in planting some plants? Maybe if he watches something grow from seed, and learns to appreciate how things grow and how fragile they are, he might learn to "respect" the plants a bit more??? I dunno - we planted marigolds with DD and she is very gentle with them (to date...), also she loves to stroke the new baby leaves of the frangipani cutting we're growing. Does he have his own garden? We each had a vegie / herb / flower garden at home when we were kids, and I remember it being good fun watching things grow and learning what is a weed etc.

Sorry - maybe no use at all, but just thought I'd throw it out there.