Tap his shoulder, not his butt. When you tap his butt it means you are asking him to stop.
I think what you mean by this is that a tap of the whip/leadline on his hip would mean "step your hiney over , disengage and stop".
Do I read that correctly?
I haven't read the whole thread, so forgive me if I repeat what others have said. Trying to lunge a horse and having the horse turn and face the handler is a VERY common problem. The handler then tries to circle aorund to get access to the horse's side in order to drive them forward, and the horse keeps moving the hind end away , facing the human and soon the horse is "lunging" the human.
In lunging, you want to stay somewhat anchored in the middle. When I lunge, I do walk a small circle, almost pivotting on my inside foot, while facing the horse. My outside foot's toe will be pointed at the horse's shoulder.
So, your problem is to get the hrose to give you it's driveline (the area where you leg would normally aply a little pressure to say "forward"). If the hrose persists in facing you, you have to get it to move its' face/shoulder out away from you, out onto the circle, right?
So, start by putting pressure on the inside edge of his face (like, aim for his inside cheek/nostril). Applying pressure means kind of "tapping" the air with your hand, or if the hrose is dull, swingingthe lead line or even vibrating the whip in the air-pointed right at that spot.
You want the horse to move away from you with his front legs. You want him to step sideways with his outside front leg, onto the track of your imaginary circle. When he does, you will be able to shift the focus of your driving pressure furthe back toward his driveline.
Also important is that you keep a forward leading pull on the line with your leading hand. You want the horse to be thinking in the direction you want him to move. So, put some feel on the rope and even a few tugs. Watch his eyes and ears to see if he is looking in the area you want him to go. When he does, support that with a soft drive on his drive line. So, he's thinking forward, and you get his feet to follow his thought.