Half halts help a lot. As soon as your horse lifts his head immediately loosen the rein as the reward. Eventually with patience it should help
I find what worked for my horse is using more thigh and lots of bending serpentines and circles. Also I found holding my rein for just a few seconds like 3 seconds at the most and then letting it go it helps them learn that you won't keep holding there head and that you'll let go.
Lots of half halts. Lots. Lots. lots.