Why do you need to stop him chewing on the bit? He may need to do this to be relaxed and comfortable.
It is important that once you have come to a halt you give the horse loose enough reins that there is no contact on his mouth. He did what you asked; came to a halt, now you must let him have his reward, which is NO pressure on his mouth.
If you feel uncomfortable, like you are worried he'll just take off all of a sudden, then you might want to work on learning how to rapidly shorten your rein when needed. That way, if he does start to move suddenly, you can quickly take back the slack in the rein.
After you have more time in the saddle you may come to worry less about that becasue you will feel secure enought to know that if he does move of his own, and maybe a bit suddenly , you can easily deal with it and won't come off.
It's important to not "ride the brakes" so to speak by always having a hold on the horse's mouth , even when he is just standing around, becuase you will dull out his mouth, and then when you need him to have brakes, he won't have them anymore.