I use annoying taps on the same spot on his hip. Use a stick no less than 4' so you don't get kicked. Try to stay in the saddle area. I give a light tug on the halter, just a signal, then if he doesn't lift his head, start tapping, enough that he feels it but not hurt. The moment his nose comes off the ground, put the stick/whip to point behind you. Start walking with him and as soon as he nose dives, ask again and start tapping. Be patient and do it until a touch on the halter will bring his nose up and it will stay up. You must teach this from the ground first. When you ride, a light touch on the bit and start tapping a little behind your leg. Keep it rhythmic. The tapping is to annoy, not inflict pain.