Since your horse already knows how to trailer, this shouldn't be hard to fix.
Is this a straight load?
Walk up with him to back of the trailer--have the lead rope clipped on the side of his halter instead of the bottom for more control. Step into the stall opposite of the one you want him in, and urge him to enter his stall. If he doesn't move, just wait for a minute, then ask again. Wait and ask again. Wait and ask again. Hum, sing, look off in the distance, think of other things--maybe do the multiplication tables in your head or something. Wait for him--he has to know there's no choice, he has to give in. He'll get bored very quickly and load. Be calm, patient, don't be in a hurry--tell him you have all day, and mean it.
Don't back him up or circle him or anything else. Make him quietly stand there until he loads. If he gets halfway on and backs out--that's fine, let him--just stop him right when his front feet hit the ground.
The important thing is this, other than backing out of the trailer itself a time or two--there's no back or sideways, only forwards.
But don't force him, wait him out. After 10 or 15 minutes, he'll get bored, and he'll load. Every time thereafter, it'll be a shorter time, until he just jumps in.
Arrow did some half rears, too, and I just took a single dally around the back pole above the partition right in front of me at the back of the trailer so that I could tighten it quick--if he reared or pulled back, I could hold him because he was just pulling against the metal strut. Once he's quiet, you can loosen it again immediately. When he walks in the trailer, just let the lead follow him in and it'll slide off the pole.
It sounds like once he's in you're fine--but for anyone else reading this thread--make the horse stand quietly, when he's quiet, get out and put up the chain or butt bar or whatever, close the door, then go around front and tie his head. Never tie his head with the back door open--if he gets his back feet out with his head tied, you'll have trouble!