If you have a pelham–one with an unbroken mouthpiece and a slight port–try him in that without the curb chain and without a curb rein. It hangs in the mouth like a boucher and will offer tongue relief. While not something for long term, it might give you direction on where to go next as it offers several different things–lots of stability with the fixed cheek, tongue relief with the port, it’s up off the bars, and a stable solid mouthpiece.
This is the kind of bit I’m talking about. This is the traditional pelham I’ve always used. If you have a single jointed pelham, don’t bother–you already know he doesn’t like a single jointed bit.
If you find he likes stable, a micklem bridle is also a good option. Paired with a mouthpiece he likes and a stable cheek, it offers a LOT of “stillness.”