OP Also apologies for going off track
Either roundings will have much the same effect. Having someone ride him in 2 reins that doesn't know how to handle them could be worse than using roundings as they could as easily end up with all the pressure on the bottom rein. A Kimberwick actually has less 'curb' effect because it doesn't have the long shanks and the action on the reins is closer to that of a snaffle if you can find something like the one I'm posting a pic of.
The ideal is to get him back into a snaffle and to do this is probably going to involve going right back to the basics others have suggested so he learns the 'whoa' cue from the ground which can then be translated under saddle. Half the horses that end up in strong bits are that way because whoever trained them missed out that vital part of the educating process and think it can be taught by heaving away on the reins, the horse gets confused and the first reaction it has is to throw its head up to avoid the contact and/or pull against that pressure like a game of 'tug of war'
I don't have a problem with martingales used in these situations as a temporary tool though I wont ride in a tie down (standing martingale) as I like the horse to be able to have the ability to raise its head to right itself if I allow it should it trip or lose balance - something you can do with a correctly fitted running martingale Endiku
I know pretty much nothing about western shanked bits and some of them scare the crap out of me and I would be terrified to ride in one simply because I know nothing about them. There are exceptions of course and some bits were never meant to be used but to an experienced western rider on a well educated horse I'm sure the ones that worry me aren't as bad as I think they are
The length of shanks in European hacks even differs over there - probably so people have more choices. The UK has fallen into the same trap as everywhere else over the years where harder and harder bits are used to replace good riding and training skills, the problem is that quite often once a horse has been used like that its very hard to get it back to where it should be because they just get a harder and harder mouth.
Most of the showjumpers that use them do so because they find themselves with horses that have difficulties with bits for some reason or another. Given the success that people have with them I can't agree that they are wrong - the Olympic Silver medalist in London actually rode his horse in one - to be competing at that level you can't possibly risk having a horse that's confused - far too much at stake. If a horse can perform like this in one then it can't be that wrong
We will have to agree to disagree https://www.youtube.com/watch?v=ap3r9cVKN2k