You can find lots of videos on youtube for trailer loading. Without the trailer, what you need to work on are your go forward and back up cues. And going forward over stuff that the horse doesn't like is especially helpful. There's a thread on this board about driving a horse over a tarp and there were plenty of folks who saw no utility, but being able to drive your horse willingly over something he doesn't like has plenty of utility here. If your horse doesn't have the confidence to go over a tarp for you, how much confidence will he have going into a trailer?
Don't use food to coax him. I've been going to shows for quite a few years, and at the beginning of the show season there's almost always someone who can't reload at the end of the show. In every case they've brought grain and a bucket with them. Comments like "I've been feeding him in the trailer every morning and night for xxx days" will be spouted. But the horse was never trained to go in the trailer. That's kind of like saying you've trained your horses go into their stalls in the morning to get fed. It's not an impressive trick and it only works when hunger is stronger than fear. You might have a very long wait.
You can give him food in the trailer after he is loaded, but the food isn't there as an enticement to get him loaded. It's to make him more comfortable after he has loaded.
Getting him loaded at home doesn't mean he'll reload when you are far from home. Ride him somewhere not too far from home and have somebody drive the trailer to you. You can see how he loads in an unfamiliar location. If he won't reload, it's not a disaster and you want to know that before you've gone miles and miles.