I am not sure what kind of bit you use, but the first thing I would do is make sure the bit is not too low in his mouth. If you ride in a snaffle there should be at least one wrinkle in the corner of his mouth. If this doesn’t help there is something you can buy that attaches to the bit. It is rubber and lays against the tongue. It also helps with horses that displace their pallett. I am not sure what it is called, but you should be able to find it in a tack supply website. It should only be a few dollars. I had a horse that would play with his tongue when he was ridden. Sometimes he would become so engrossed in doing this that he would almost fall down. I rode him with the rubber thingy for a while, and he totally stopped messing with his tongue.
The bit may be too low in his mouth, so get this checked and adjusted if necessary. Also, a horse that puts his tongue over a bit is probably tense and not accepting the contact and bit. You could try a copper roller to encourage him to salivate and play with his bit and accept it, or use a mouthing bit, which has movable ‘tassles’ on the link or centre of the mouthpiece to encourage a horse to play with the bit. Once your horse is accepting his bit, get hijm to work forwards and be listening to you as he will have to concentrate on what hes doing with his legs and body rather than whether he can get his tongue over his bit. A flash noseband may help as he wont be able to open his mouth to get his tongue up and over his bit, but i would advise finding the cause of the problem first before trying to battle with him over it.