As soon as his head dips just a tiny bit, take away your hand from the poll and/or release the line. Doing this lets your darling now that he won the game and got the right answer.
Do this until he knows for sure what you are asking for, (It'll only take a few minutes or less) Then ask him (don't release) to keep his head down a bit longer. then a bit longer. then a bit longer.
A good horseman should not need to stand on a chair or stool for ear trimming, or have to tiptoe to put the bridle on. A really good horseman is able to bridle his horse while sitting in a chair or while kneeling and/or trim his ears with no reaching at all.
It takes a little practice and time but not very much.
Also, when the horse lowers his head he generally calms down and it shows he trusts you.
Adventure Horse Riding in NYS ......www.RideNYS.com