J.K.
Every time he climbs up there, tell him "No, do not climb onto the counters" and remove him immediately. Do this EVERY TIME he does it. He is perfectly capable of understanding you, he just wants to do it and will try to see if you will change your mind. Can you blame him? How fun would that be?
Be consistent about it and he will eventually figure out that you mean business. This situation is too dangerous to ignore and should be dealt with immediately.
Good luck! (I have a 20 month old as well and he tries to get away with this stuff too!!)