Try bit butter to make the horse accepted the bit more or have you tried bitless
Depending on the horse of course, but something I like to do with stubborn horses is hold the reins tight giving half halts until the give and take the contact, as soon as they give you want to as well so loosen your reign pressure. It's basically just rewarding them by giving them their head, if you carry on this process with every time he stops taking the contact soon enough he'll understand that if he is in a contact he won't have the constant pressure.
Half halt! Inside leg to your outside rein for transitions and also if his head goes up
Keep ur reins loose and kind of wiggle the reins
Try just do nothing. Hold reins on pretty short long and keep your horse active, he should go on the bit and then you can play whit him to make contact more flexible, and do many transitions and exercise like ride on circles or 8 :))