putting the bit in

I've read all the post on putting the bit in your horses mouth by putting your thumb in the corner and opening the mouth and sliding it in while holding his head lower with your right arm.
Here's my problem, he's 5 and has been in training for about 3 months (3 days a week) He fights it, after its in he's fine. He's head is up in the air or on the ground and I'm having a hard time keeping up with his head.
I didn't mention that he's 18.2 hands. I've tried the molasses once. I put it on the bit stuck my thumb in his mouth to get him to open and some how I had a bitten finger. He's bit is 6.5" snaffle so its big for my hands to begin with. I just can't figure out how anyone can get their horse to just suck the bit in their mouth.
I'm up for any advice.
Thanks

Try leaving his halter on under the bridle, that way you have better control.

Hmmmm..... my old horse was 18 hands. I had no problem with his bit. What is his favorite treat?

He's a food hound. He loves everything. I've wrapped a fruit roll up on his bit, and he loves it once we get it behind his front teeth. He just doesn't want to open his front teeth.

When you're putting the bridle on, be sure to NOT hit his front teeth with the metal. Be careful to put the bridle in slowly, so you don't accidentally hit his teeth with the bit.

I'd also have the vet out to see if his teeth need to be floated. Have his wolf teeth pulled too.

Sounds like you need to work on getting him to respect you when you ask him to give you his head. Presure and release. Place your hand on the top of his head and once he drops his head release do again and again untill he just drops it for you then incorporate the bit.

I do the bit cookie method. Bit goes in then cookie every time my horse gets the bit. Its the only time she gets a cookie so I don't think I'm spoiling her.
I have a video somewhere....I'll post it.
I know I should have done the throat latch first but oh well.... Notice she is tied to the hitching rail although she would take it the same no matter if she was tied or not.

You need to teach him to put his head down on command. Pressure and release like KTSpeedhorse said. He's WAY too big to not know that and needs to respect your size to his. You can even use a lead and halter and just ask him to lower his head bit by bit. It could be a bit of a process being that he is so big its not easy for you to just put your hand between his ears and hold his bridle and wait, I think you would be floating.. lol...but teach him the "head down" command either verbal or just by asking him with your hand or lead and then work up to bridling. Have you had his teeth done? This may be something going on as well if you havent..

You need to teach him a 'drop your head' cue, like has been stated. He's learned he can raise his head and evade getting the bit in his mouth.

I completely agree with everyone about getting him to lower his head on cue and to leave it there, with repition this should stop him from raising his head when the bit comes close. Then as far as getting the bit in it sounds like you have the right idea, I have read (I think it was John Lyons) to train them to open their mouths useing a lead rope in place of the bit, that way there would be no problems with hitting his teeth and they learn to open wide. I also agree with getting his teeth checked and maybe removeing the wolfe teeth if he still has them.