I think I would go straight for a pelham to be honest - it made the world of difference with my pony. You may well be able to pick one up quite cheaply second hand as they are easier to find than hanging cheek bits. You can always knot the curb rein at a fairly loose length while you get used to it. When I was learning how my pelham worked I used to ride (in a school) with one rein at a time so I learnt how each one worked. I'd never ridden with double reins before but found it surprisingly easy to manage once I got used to the idea and got brave enough to try!
Just a thought, but it may be that he doesn't like a jointed bit. My mare hates
anything with a joint but goes very well in mullen or cambridge mouthpieces. I had her in a kimblewick for a while before we discovered the pelham and she was much happier than in her double jointed snaffle.