But a bridle on and take away the reins and give them a feed and hay to eat with the bit in x
What i would do is to lounge the horse with side reins to make him get his head down and on the bit.
Also work his flexibility while kepping him calm and steady.
Next I would I would ride him and do several exercises to make him bend is neck and get on the bit while keeping a good relax rhythm if he continues to resist your hand aids try using draw reins in warmup.
I hope I made myself clear and helped you good luck!