Horse trailer problems...

My paint gelding has not left my house since I got him over a year ago! I have tried everything to get him in it. I have fed him in it (he panicked and choked once, so we won't do that again), put a rope around his butt (that ended badly lol), literally everything in the book. I have been working with him and he will get in, but after about 10 seconds its just to much and he backs out very fast. I have also been getting to realize outside of the trailer he has to work, inside he gets petted on and gets treats.
He is supposed to be my drill horse, and drill starts up in the next couple weeks. I can ride my older mare at the first couple practices, but I really need him to be there so he can get aquainted with the other horses and the feeling of an arena. I would rather not have to ride my poor old mare all year in drill again!
Should I have someone shut the door before he backs out? Would he probably panick really bad? I really need help!

Load one foot then back him out and when he is comfortable with that then load two feet and back him out and so on until he loads all the way. Once he is in don't just slam the door and take off. Let him stand, load him a few more times then out him away. Make sure you back him up BEFORE it is his idea to back up.

Also, they sell loading helmets (made of leather to shield head from injuries that could occur from rearing etc) and break away halters (the part that goes from cheek over their head is leather so it will break instead of them getting hurt.) Worse case scenario you could always lightly ace him (gentle tranquilizer depending on cc's given.) Its like the people who have to pop a xanex before getting on plane. Granted that's the last option if NOTHING else works.

You need to work on this every day since your on a short schedule. Once you have him in and comfortable, do short trips starting with just a few yards working up to miles. I always say its not the loading that's hard on the horse its the riding time. If that time has been traumatic in the past you need to unlearn that.
Once he's going in and comfortable start feeding him in there every day. It needs to be a safe place in his mind.
Do as Kevin said and do the one foot then out 2 feet then out. Make it your idea to back out not his.
You need 2 people, one on the lead rope one driving with a whip from behind. Use a rope halter and while one directs with the rope the other drives. The driver puts pressure on from the rear by slapping the whip on the ground until the horse steps forward. With every effort by the horse the driver takes the pressure off. Horse steps forward, praise and stop whipping. Wait...Do it again and again and again until the horse is in the trailer.
I'll give you my worst case on a horse we used to have that we, like you, tried everything. We used a stud chain on a lunge line that we ran out the front of the trailer, then the chain was placed across her gums so every time she pulled back the chain tightened on her gums, she moved forward the pressure came off. Didn't take long, about 10 min. She was in the trailer.
I know that sounds really bad, I'd never do it again. She never gave me any trouble again about going into a trailer either.

I second this very strongly; you need to work with his comfort level first, NOT try to keep him in there really long right away. Load him, unload him immediately...

Dennis Reis just had a trailer loading special on RFD...he basically makes it more the horses idea, by working with his comfort level and natural curiosity. Now his idea was nothing new to me, as I've used similar tactics before, but it's kind of like what Kevin described...

You would start out with a plat form near the the trailer, get him to send over it back and forth, til he is comfortable with it...then stop him on it for a second or two, and let him step off, etc...

Then move the platform closer to the trailer (if your trailer doesn't have a ramp), and do the same thing, til the platform is right up to the rear of the trailer; he can use it as a step into the trailer. Have him stop alot on the platform and encourage him to investigate the trailer. He will eventually step in quite easily, once he realizes you're in no hurry, and aren't going to force him in.

When you have him stepping up to the trailer confidently, start asking him to step into the trailer; starting with one foot...get the foot in, and then back him out, and work from there.

Agree, you dictate what's going to happen, he doesn't get to call the shots. So often we're so intent on getting them completely in the first try we don't take the time to break it down into small steps.

Should I place another horse in the trailer for the first few rides?

If you have another horse that is calm in the trailer, I think it would be a great idea.