Is this what your gelding looks like when he's trying to get the bit out of his mouth? If so, he hasn't learned to accept it.
Excuse that bridle, it was my junk bridle that I use to get new ponies used to being bitted. It fit her terribly. This is my mare as a four year old, being mouthed. This was her 2nd time being bitted and she was still learning what to do with it. What I did, is I would offer it to her with a sliver of stredded carrot (her favorite treat) under it, so she could only take the carrot if she took the bit. The second the bit went in her mouth, so did the carrot. I would leave it for a few minutes without messing with her, then take it off. The second she released the bit, another piece of carrot went into her mouth. I only did that the first few times, just to make sure she understood the bit wasn't bad. Make sure you don't hit his teeth with the bit. He might be reacting to that.
I also spent a lot of time lunging her while she wore the bit, with the lunge line attached to her halter, not her bridle. My trainer told me to make the bridle become just 'another thing' that meant we were going to have a normal day. I even went out to catch her with my bridle instead of the halter sometimes, and we went on trail walks with it on, groomed with it on, etc. Soon she realized the bit didnt mean scary new things, and she started to even look for the bit when I came to get her.
At that point, I had already done all of her ground work. She already knew to yield to pressure on her poll, nose, etc- so I then taught her to yield to pressure on her mouth. If I applied light pressure (on the ground) to the right, I waited until she moved her head a little in that direction. Then I released. I did this in all directions, Back, up, dwn, side to side.
I honestly think your best bet after that step, is to find a trainer to help you. it sounds like your boy has a lot of gaps in his training.