Just because your horse rides in a western curb it doesn't mean you should go to an english curb type bit. Western curbs are to be used neck-reining only, so switching to a curb with contact (english) might make the transition more confusing for the horse.
Here is some information I typed up:
For experienced riders only. Uses curb chain action on the horse’s chin. The bridle attaches to the uppermost ring, the snaffle rein attaches to the big ring, the curb rein attaches to the little loosering. The idea with the Pelham is to ride on the snaffle rein 98% of the time, and just “tweak” the curb rein as needed for a little extra “listen to me” power communicated to the horse. A relatively mild bit when used correctly, by that I mean the rider should stay on the snaffle rein only most of the time; the curb rein should not be used in excess. Many riders misuse this bit and/or have a hard time figuring out 4 reins, which is why I labelled it “experienced riders only.”
For experienced riders only. Uses a curb chain on the horse’s chin. The bridle attaches to the small vertical-type ring at the top, then the reins attach to either of the slots. This bit is much less refined than a Pelham, when it comes to discussing English curb bits. The problem with the kimberwicke is that you have no refinement, no snaffle rein, therefore no relief from the curb action of the bit. This can be quite bothersome to horses that don’t need the curb action on a bit. I much prefer a Pelham.
Hmm, I wonder if it's just that he's not used to the contact associated with 'english' riding as opposed to western? In which case something like a Kimberwick/Pelham will only magnify that contact ten fold. But your trainer knows the horse, knows you and knows what pursuits you are looking to accomplish so try what she suggests first. If that fails, I'm sure she will tell you that a process of retraining is in order to teach him to accept a soft contact with a softer bit. :) I hope you didn't think I meant any disrespect by saying try a snaffle first. Just my opinion :)
My horse hated a snaffle type bit to and I used a Pelham for a long time, my Pelham had a rather large copper port, and I found my horse was waaaay softer then before. So recently I tried an experiment, I got a medium wire snaffle that was made of copper. My horse is even more responsive to this bit then he was to the Pelham. I think the copper really made a difference and I suggest maybe you try a similiar experiment :)
RoyalsRebel - I know exactly what you are saying. =) Trust me, going back to a snaffle was the first thing I tried.
JDI - His teeth were done just a couple of months ago and I'm about 98% sure it is not a saddle fit problem as I have no issues with him when I ride him in his curb with the English saddle. I have tried a single jointed loose ring, a french link loose ring, and a french link eggbutt per some recommendations on here as well as from my trainer.
Void - He seems to like the taste of sweet iron bits over stainless steel, I have noticed that. He accepts the bit better. But he's still no better under saddle.
Any suggestions are welcome. I'm kind of at a loss at this point.
Don't expect to change from western to english and have it be 100% right off the bat; it's a completely different way of riding. Your horse needs to learn to accept contact and direct reining, and that in itself is a boatload of training. I would pick a bit that he seems ok with and get down to the root of the problem - training - rather than the "surface" problem - the bit.