That's easy! Start by sticking a treat to the tree or any other vertical surface at his nose level. Have a command for him to go get the treat. Get something a bit moist that will stick on it's own, like a piece of wiener or cheese. Then you gradually get the treat up higher and higher so she has to reach to get it, even stand upon her hind legs
with his front paws on the tree to reach the treat. Then even higher and have him run to the treat, so its run, jump up a bit to get the treat.
Then go back to just above nose level where he has to hit the tree with paw to pull himself up (like waist level). Have your dog sit and wait. Go up to the tree or any surface you are working with, and pretend
to stick a treat to the tree. Keep the treat hidden in your hands. Move back to where your dog is, give him the command to get the treat. He will get up on the wall looking for the treat - after a sec or two, he will look at you wondering where the treat is - praise and throw him the treat - do this over and over, until he knows that he has to run to the three, pull himself up, turn and look at you to get a treat.
Then instead of throwing him the treat, stand closer and show him the treat in your hand, move backwards so that he has to come off the tree and follow you to get the treat. You now have the sequence: run to tree, jump up on tree, turn and come back to you.
It also helps to have a very food motivated crazy Malinois.