I was going to suggest a myler type bit as they are usually quite thin, shaped which helps the bit sit more comfortabley, and they have a wide range of different mouth pieces what may suit your neddie xx
Thanks all. I have tried him in a Myler but there's still not enough room - he really does have a very large fleshy tongue! Was thinking of something with a port but they're not dressage legal so am feeling rather at a loss. I'd like something that doesn't put pressure on the tongue and can fit in his mouth easily so I guess it'll need to be thin.