I think I am going to slice my son's tree ladder so that he comes back down to the ground.
While he is in the treehouse? How will he get down?
He is grounded so he needs to go inside. I could care less what happens after slicing this ladder.
If he doesn't have the ladder he might get hurt trying to get down. There's no reason not to wait until he is not in the treehouse to start dismantling it.
Oh, so I can't do anything?
While it makes sense that you are upset with your son, do not put him into danger of injury. You are better off finding a safer way to get him to come down, or maybe just punish him further. It may also be a good idea to hear him out because odds are that he is pretty upset for him to be hiding up there like that.