You can always put his butt to a wall so he cant back. My horses always put their head down between their legs, bend their legs some and thats it if I simply use a treat and nothing else. To get them to actually go down on one knee, I use a pully system. You have to lift one of the front feet while asking them to go down for them to do a proper bow.
You have to be SO careful when teaching to bow. I tried holding the foot up and asking down, but when the foot touched the ground I'd have to let go, or get pinched. When I let go the horse would automatically come back up, or I'd accidentally drop its foot and scare it.
So, I put a rope around the pastern, looped it under the belly and over the back so I could grab the end. If you need to release you can easily just let go, but it gets rid of the danger of your fingers being smashed.
Don't ever force the horse to drop down. Don't try to push them hard or upset their balance. They could fall, hurting their knees, or you. Bowing generally takes quite awhile to get through to the horse. They aren't used to doing it, and they have to learn how to position themselves to do it without falling. Just lift the inside front foot with the pulley system and ask them down with treats and pressure. Go slow, and reward them for the slightest try.