what can i do to stop my horse from 'tounging' his bit?

What type of bit is it?

If he likes to play with the bit, you might want to try a bit with copper rollers in it, a lot of horses enjoy those bits.

Is this a new habit, or has he always done it? If it's just started, perhaps the bit is worn in such a way as it has become uncomfortable- the metal does wear down over time. If that's the case, get a new bit. You should also have the vet check his teeth- if they are in need of floating, that could cause the bit to be uncomfortable when it wasn't before. If he's always done this and the vet has checked his teeth, it isn't going to stop. You can get a bitless bridle or hackamore, or work with it and get him a bit with a roller that he can play with.

My gelding loves his roller bit it is just a simple roller curb bit. It really keeps him busy. Also if he puts his toungenover the bit you can get what is called a cavason And it just goes on under the headstall and prevents him from his tounge going over the bit.

Try a bit with a roller... That'll give him something to play with.

he probably likes the bit and if he does this while he is riding, tap the reigns and tell him no. if he is doing this while he is standing there he is bored and gives him something to do. or........ check his teeth. they may need to be floated and they are rubbing his gum and causing him pain.