First where is your horses favorite place. For example if he likes the pasture park your trailer in the entrance of the pasture gate. Open up the doors and try to lead him in so that he has to think that the way to get to his favorite place is through the trailer. I did this with my gelding who was trailer shy. Another idea is to lead a his horse buddy through the trailer and have him follow behind. He just needs things to reasure him that the trailer is a good and freindly place and it wont hurt him. Reward him with one treat when he goes in the trailer. Not before because you would be rewarding him for being scared but only reward for when he accomplishes something that you wanted him to do. Best of luck hope you can get him in the trailer. Ps. if you you have linking together metal gates the people use to put up a quick round pen somtimes then you can put that around the entrance of the trailer and have entering the trailer the only option to get out into the pasture. You might have a treat in the trailer while he goes in.
Don't use treats to train! It creates a demanding horse. We round-pen train. Back your trailer to a round pen, open the door to the round pen. Keep the horses feet moving, when the horse decides to go into the trailer, allow it to rest and stand still ONLY in the trailer. If you do not have a round pen then lead them to the trailer and you hop in first. Allow them to sniff around, this will take alot of time and patience. Gently tug on lead, eventually your horse will give in to pressure of the lead, immediately release pressure, keep doing this till they go into trailer