Okay then id work with a transition bit. Has the horse ever used a curb bit before?
If not look for a transition bit. Most horses end up loving having a medium ported bit because of the tongue clearance and so a transition bit can help with teaching them the neck reigning in the short term.
At first a curb bit will be confusing to a horse that has never used one. So in the short term it will be necessary probably to give the neck rein cue and reinforce it with direct rein until the horse understand what your going for.
Good luck finding the right bit for your pony. Im still having trouble finding one for mine because of his sensative mouth.